Which actor played chips best friend Mike on my three sons?

You might have been a bit mistaken here. Mike was the original oldest brother in "My Three Sons" as played by Tim Considine. He was written out of the series in 1965. Middle son Robbie suddenly became the eldest and Chip's friend Ernie, an orphan, was eventually adopted by the Douglas family, re-establishing the "three sons".
In the very early Black and white episodes )actually the first three seasons) Chip's Best friend was Hubert Pfeiffer, nicknamed
"Sudsy" and was played by Ricky Allen.
Hope that helps!