Tim Considine left by choice, feeling he'd outgrown the role,
although it is to wonder (as Bugs Bunny might say) why he
didn't come back from time to time (as, for example, Vivian
Vance made an annual appearance with Lucy after 1965).
Ernie was an orphan friend of Chip (they are, of course,
brothers in real life) and to re-establish the "three sons"
Steve decided to adopt him.
The idea of Robbie and Katie having triplets (all male) was
a hedge against Fred MacMurray's leaving the show, which
never happened. I'm afraid that had the show gone in that
direction it would not have made it, with shows like "All In
The Family" coming in, and Don Fedderson was not exactly
Norman Lear.