Susan Oliver (1932-1990) played a love interest for Steve, but also caught Robbie's eye as a very desirable older woman...!
http://www.librarising.com/astrology/celebs/images2/S/susanoliver.jpg

Jon Walmsley (b. 1956) wasn't a star yet -- he went on to The Waltons as Jason. Late in the series on M3S, his character was a young guy Barbara thought should have his hair cut:
http://statici.behindthevoiceactors.com/behindthevoiceactors/_img/actors/actor_4708.jpg

Vera Miles (b. 1930) played a character on several episodes involved with Ernie's whereabouts leading up to his adoption. Robbie encouraged Steve to romance her:
http://ecx.images-amazon.com/images/I/51PVrxy%2BYxL._SY445_.jpg

Young Erin Moran (b. 1960) had joined the cast of Daktari prior to her guest spot on My Three Sons, Season 10, as a bully at Dodie's expense. This little angel certainly didn't look the part of a troublemaker:

http://daktaritvshow.files.wordpress.com/2013/06/erin-moran-season-four-2.jpg

Recording star Jackie DeShannon (b. 1941) was the embodiment of Robbie's main interests: lovely ladies & music...! Was she maybe "the one that got away" prior to Katie's arrival?

http://upload.wikimedia.org/wikipedia/en/e/ea/Put_a_Little_Love_in_Your_Heart_(album).jpeg

"Just a note that Fred MacMurray was born in 1908."

To quote Futurama: "They say I'm robbing the cradle, but I say she's robbing the grave"

Remember soon after Rob met Katie, and Katie's sorority sisters were giving Steve the once-over, checking him out? These were supposed to be young women generally no older than 22!!! At that time in real life, Fred M. was approaching 60!

Thanks for the replies, fellow M3Sers.
************************************************

David White (1916-1990) appeared as a father in the black & white era, eventually moving on to his most remembered role as Larry Tate on Bewitched (alert: Try not to miss his episode of The Untouchables, with Liz Montgomery at her sexiest):
http://3.bp.blogspot.com/-2MnHYqVs568/UiJ8e2W7PSI/AAAAAAAAA1Y/VCFY-OLdAR0/s1600/David+White-Peter+Gunn.jpg


As long as we're in the black & whites of M3S, let's mention Ann Morgan Guilbert (b. 1928), who played a neighbor in a hectic Season 1 episode -- she would soon become The Petries' neighbor Millie on The Dick Van Dyke Show:

http://d2948ivoqgnz9i.cloudfront.net/photos/154201.jpg

Back to the color seasons:
Yvonne Craig (b. 1937) was a motorcycle-riding meter maid on M3S prior to her big role as Batgirl on the Batman series. Steve discovers that her character is seeing Robbie, but despite the connection, she still gives him a citation:

http://elviswomen.greggers.net/images/craig062.jpg

Micky Dolenz (b. 1945) of The Monkees pays a visit to the Douglas household late in the series:

http://ichef.bbci.co.uk/images/ic/544x306/p01br05n.jpg

Sebastian Cabot (1918-1977) had a small recurring role as Sally's father during the black & white courtship of Mike (Tim Considine) and Sally Morrison (Meredith MacRae). Of course he went on to become one of the stars of Fedderson's Family Affair:

http://img1.bdbphotos.com/images/orig/b/e/bem4fynx0rruny0m.jpg

Doris Singleton (1919-2012) guested in both black & white and color, notably as Sally's mother and, much later in the series, as the mother of Polly Williams (Chip's new wife). She's probably best known as Carolyn Applebee of I Love Lucy:

http://2.bp.blogspot.com/-kJzZTLi15Vw/UKgFgpz3KII/AAAAAAAABMo/ziiinKlR0UA/s400/Doris+Singleton2.png

Jeremy Clyde (b. 1941) of "Chad & Jeremy" played visiting British music sensation Paul Drayton, a character who also knew a little about motor scooters. It takes some time for Chip & friends to find out what a talent they've been hosting (color seasons):

http://www.johnhora.com/Jeremy%20Clyde2400.jpg

Susan Silo (b. 1942): Her lovely French character's job includes giving Robbie a tour of Washington, DC. Rob's not particularly interested in the sights, but he later becomes quite excited when he sees her in an evening gown...!

http://www.sitcomsonline.com/photopost/data/785/MyThr7SusanSilo.jpg

Lori Martin (1947-2010), child star of the National Velvet series, and Jerry Mathers (b. 1948), "The Beaver," appeared together as Rob & Katie's next-door neighbors when they moved into an apartment complex with the triplets (Season 10). Jerry had a mustache at the time. Lori had also appeared a few seasons earlier in the color era, as one of the two girlfriends Rob was attempting to date simultaneously.

http://i2.wp.com/pgoaamericanprofile2.files.wordpress.com/2013/05/lori-martin.jpg?crop=0px%2C100px%2C1227px%2C739px&resize=487%2C292
http://www.ang.af.mil/shared/media/photodb/web/2012/09/120914-Z-JV725-022.jpg

Fun thread! Here are some more guest stars to add to the list:

1. Tony Dow (1945-) This is one of my favorite episodes! The boys think Tony is a juvenile delinquent. "Guest in the House" (b/w)

2. Zsa Zsa Gabor (1917-) - "Ernie and Zsa Zsa" (color)

3. Jodie Foster (1962-) - various episodes from 1971-1972. (color)

4. Sherry Jackson (1942-) - "The Wheels" Another favorite episode of mine! (color)

5. Angela Cartwright (1952-) - Two episodes between 1965 and 1969. (color)

6. Irene Ryan (1902-1973) - "The Romance of Silver Pines " (b/w)

7. Ed Begley (1901-1970) - "The Romance of Silver Pines" and "The New Room." (b/w and color)

8. Ed Begley Jr. (1949-) - "The Computer Picnic " (color) - One of his first acting jobs. (color)

9. Sal Mineo (1939-1976) - "The Liberty Bell" (color) - Nick at Nite showed this episode during their 10th anniversary marathon.

10. Martin Sheen (1940-) - "The Guys and the Dolls" (b/w)

11. Butch Patrick (1953-) Various episodes between 1962-1971 (b/w and color)

12. Mimi Gibson (1948-) Three episodes between 1966-1968. Her last acting credits. (color)

13. Charles Herbert (1948-) Three episodes in 1966. (color) Mimi Gibson and Charles Herbert were in "Houseboat" with Cary Grant and Sophia Loren.

14. James Stewart (1908-1997) - "Robbie Wins His Letter" (b/w)

Of course, after I submitted my post I remembered these:

Joan Blondell (1906-1979) - "Office Mother" (color)

Beau Bridges (1941-) - Various episodes between 1960-1964. (b/w)

Sandra Gould (1916-1999) - "Adventure in New York" and "Top Secret" (b/w) Another "Bewitched" alum!

Annette O'Toole (1952-) "The Chaperones" One of her first acting credits. (color)

Jaye P. Morgan, singer & actress, 1931 - present. Appeared in two episodes in the series

Alejandro Rey (1930-1987) from "The Flying Nun" was in "My Son, the Bullfighter."

Maureen Mc Cormack also guest starred!!

https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcQYNFoG6nCyqA1uqdD0KNvyDofLrWEpQ_jsYRRrWPZbxWSvW8WJ

Buck Taylor (1938-) played Newly O'Brien on "Gunsmoke." He played a friend of Mike's in "All the Weddings." He plays a father in this one and I swear he holds a baby like a sack of potatoes.

Morgan Brittany (1951-) played Katherine Wentworth on "Dallas." She was a girl Chip didn't like.

Billy Booth (1949-2006) played Tommy, Dennis' best friend on "Dennis the Menance." He was in two episodes, one in 1963 and another in 1967. His appearance as Jack in "The Computer Picnic" in 1967, is his last acting credit.

It's a amazing list of guest stars.

http://stanleylivingston.com/id76.html

Yvonne Craig, a gal I used to drool over, has unfortunately gone to that Big Batcave in the sky. She was really a hot number!

And I see nothing wrong with hot 20 year old babes lusting after a 60 year old dude in his prime.:o Sounds just fine with me. Now where do I meet them, oh, only in TV Land, that's right.:(

Dodie's friend "Margaret Spencer" was a toddler co-star to Danny Kaye but left after she couldn't deal with the audience laughing at her.

She was also the "fat" girl "Angela Clayton" who Buffy turns into a proper lady on Family Affair.

A few of my favorite guest stars are:
Angela Cartwright
Jan Clayton

A couple of other favorites:
Irene Ryan & Ed Begley

Marsha Hunt (Aunt Cecile, S8Ep13) b. 1917, would have turned 102 a few months ago.

Did anyone mention Jeremy Clyde of Chad and Jeremy yet? He played the young British lad from Liverpool who disappointed Chip and his groovy friends because he played a soft rendition of "Greensleeves" on the acoustic guitar when they were expecting a Beatles clone. LOVED that episode!

https://youtu.be/n_PoZZZq5Wc

https://i.ytimg.com/vi/n_PoZZZq5Wc/hqdefault.jpg