1. Yes, you can put an rtx 3080 on a board from 2008 and itll work just fine, with just being speed bottlenecked by the old cpu. As for the psu, as long as its a good quality unit, the wattage is enough, even with a good cpu.
2. If youre talking about SLI, then no. only the 3090 has SLI support (3080 does not), and SLI is only possible when 2 cards are identical. (meaning 3090 with 3090 and 1070 with 1070.)
If youre talking about gpu passthrough, for a vm or something, then yes.
3. what kind of problems?
the best cpu in that motherboard is an i7 7700k, which is still okay for gaming, but is pretty slow with only 4 cores in this day and age, so it'll probably be a bottleneck in a few years.