the one thing I do not understand Why didn't Mike and Sally come to see house in Ca or when Robbie and Kaite or fore that matter when Steve and Barbara were married and why didn't they so much as send a gift for the triplet it's like Mike and Sally ceased to exsit

Mike must have done something horrible to make Steve disown him! That's why we never saw him or Sally again.

Seriously, it's easier to think of "My Three Sons" as two separate series, the first five seasons in B&W with Mike and Bub (originally aired on ABC), and the last seven in color with Ernie and Charley (on CBS). Mike is only referred to a few times early on in the CBS run, and Bub is never mentioned. I'm sure the producers never figured that the early episodes would be rerun (they were withheld from syndication until the mid-80s), so they probably thought that they didn't need to address the history of the show.