He was on the show for the adoption part and then no more.

Wilson might have been in one or two black and white episodes, too. But he disappeared without explanation after the adoption three-parter.

Later Ernie developed an allergy to Tramp, which was not resolved in the episode. But that allergy also was never referred to again.

And then there's Mike. In the first adoption episode Steve said he'd gotten a position "back East." Then in an episode where the boys all thought Steve was going to marry without telling them, the boys were going to live with Mike in some western state, Arizona, I think.

Arizona is correct, I believe it was said Mike was teaching at a university.

It all makes sense now. Wilson never seen when color epis. started, Mike gone after color episodes, ipso-facto Mike took Wilson with him to Arizona or where-ever he wound up.

Been watching My 3 Sons in order once a week every week all from DVD. Today was the first day of the color years. Now here's where some questions brought up here are answered, first, Mike is seen in this first color episode (briefly) having just gotten married and then Steve and Mike have a conversation. Mike thanks Steve for everything he's done and Steve says something like "Who'd of thought one day I'd have a son becoming an assistant teacher of psychology at a University and moving East". So Mike moves "East" becomes an assistant teacher of psychology at a University. That's as much detail as is ever given, then poof Mike's gone forever.

2nd, in this first color episode there's Ernie and he's carrying his dog Wilson in the Douglas house as he's now going to hide out there since his "parents" are moving overseas somewhere and they can't take Ernie with them. Here's the first time we find out all the times in the b/w years Ernie has called his Mom and Dad Mom and Dad now it turns out they were foster parents not his real parents. My 3 Sons must have revised Ernie's history for these color shows so they'd have an excuse to bring Ernie in as a Douglas son not just as Chip's friend. He asks that if he is transferred to another foster parent he be allowed to keep Wilson with him. Next week I'll see 2nd color show, but from memory I believe Wilson is back again at least for a while then he quietly disappears from the series with nary a mention again (like Bub).

Apparently Mike & Sally's stay in the east was fairly temporary, because in a later episode you will hear Arizona referenced as to where Mike is teaching.

The adoption episodes are the best of the season; in my opinion, the series goes into a bit of a lull until Katie's arrival in Season 8 -- though of course, there were standout and otherwise memorable eps all along.

Wilson went to live with Mike, Bub, Robbie, Sally, Sudsy, Ernie’s old parents, and the first Mrs. Douglas.

The adoption episodes are the best of the season; in my opinion, the series goes into a bit of a lull until Katie's arrival in Season 8 -- though of course, there were standout and otherwise memorable eps all along.

I agree--the rest of the 6th and 7th seasons were kind of on auto pilot--except for occasional visits overseas and Hawaii. And the ratings suffered for it I think. The 7th season was the only color season with no multi-episode arc (all penned by head writer George Tibbles) to open it with some significant Douglas event.

Good point -- here, I'll summarize, trying to add my own titles:

Season 6 ('65-'66): Adoption of Ernie
Season 8 ('67-'68): Rob's Marriage to Katie (with the Move to California)
Season 9 ('68-'69): Arrival of the Triplets
Season 10 ('69-'70): Steve's Marriage to Barbara
Season 11 ('70-'71): Chip's Marriage to Polly
Season 12 ('71-'72): Fergus' Marriage to Terri

As I've mentioned before, so many weddings -- while by contrast, sister series Family Affair had none -- despite 3 of the main cast members all of marriageable ages and romance oftentimes in the air.