If I got a second hand 3rd gen will it do all the stuff the 4k one does just without 4k content?
As
@jonblatho has mentioned, the ATV4k is the 5th generation ATV.
Did you mean the the ATV4 compared to the ATV4k?
If my TV doesn’t support 4k is there any point in getting the 4k version?
Assuming you are deciding between the 4th and 5th gen ATVs, and not the 3rd gen, functionality between the ATV4 (HD) and ATV4k mostly the same.
AFAIK, there isn't really any ATV4k-only apps.
But, I personally would recommend the ATV4k over the ATV4 due to the much better HW in the ATV4K.
Also, if you are paying retail prices, the price differences between the ATV4 and ATV4K is very small. You might as well just pay a little extra and get the much better hardware.
I would only recommend getting the ATV4 over the ATV4k if you found a killer deal for the ATV4.
ATV4 uses the A8, which I think was under powered at the ATV4's launch. So now, four years later, the A8 is ancient in a technology perspective, and while the A10X in the ATv4K is now a few years old, it is just a much better chip than the A8.
IMO, the Apple made several missteps involving the launch of the ATV4 and tvOS, and putting the A8 in it was a big one.