I've watched the first two seasons on dvd and I liked it a lot. I would like to get more but when the prices come down. Getting both volumes of season 5 would cost around $70. That's about 3 times what I would be willing to pay.

Steven Douglas, a widower and aeronautical engineer, faces various problems while raising his sons Mike, Robbie and Chip.

I highly doubt those who originally watched it.
Of course, those that grew up watching the show, particularly the early 60s episodes, fondly recall the great show.
However, let's say a viewer was 20 in 1965.
They'd be 70 today.
75 if they were 20 in 1960 and 65 if they were 20 in 1970, one of its last years.
Those aren't considered "prime" viewer demographics, unfortunately.
The older a show gets, the less likely it will be broadcast, unfortunately.
Plus how do you know My Three Sons and rarely had its reruns shown often?
Please list me where the B&W eps. have been shown since the 1980s when Nick At Nite revived them.
It's a shame they're not shown anymore.

I highly doubt those who originally watched it.
Plus how do you know My Three Sons and rarely had its reruns shown often?

Don't see how it's forgotten.
It may not be forgotten, but it's likely out of many viewers' minds as it's never or rarely shown in syndication.
Of course, the color eps. are shown (somewhat), but the classic B&Ws, the first five seasons, aren't syndicated anymore.
It's inexplicable.
Here you have a show that is considered funnier in its first five years, but it's never shown on TV anymore.
What was the reason?
Those eps. were in BLACK AND WHITE ???
My gosh. NOBODY will watch THOSE programs, right??
People sure watch
I Love Lucy, Leave It To Beaver,
the first five seasons of
The Andy Griffith Show
(which were all B&W).
This reminds me of
The Dave Clarke Five,
that popular 1960s rock & roll group from England.
For some reason, for years, the group's namesake resisted a comprehensive remastering & remixing of the group's songs (like what was done with many other groups, like
The Beach Boys
and
The Beatles
).
When he finally relented, in the 2000s, it was too late as the prime audience for those oldies (those that lived in the 1960s) was bigger and more in the "purchasing mode" in the late 1980s and 1990s.
Am afraid wrong-headed decisions by whoever owns the rights to My Three Sons will similarly negatively affect the show's memory in the public conscious.

Don't see how it's forgotten.

It's a bit of a shame Bub and Mike left, isn't it? Oh well, at least we got 5 years with them. I know Frawley didn't have any choice, his health declined and he died shortly after.

Yep. Liked Bub, Robbie, etc. Not so much in later years.