I've been using a Blizzard1260/50MHz on a 3.0 A1200 without any problems.
CyberGraphX have workarounds for running on 3.0 (IIRC, 3.1 has WriteChuckyPixels() whereas 3.0 does not), a friend of mine was running a CV64 with CyberGraphX 3 (I think) on his v39 A4000 without any problems.
If you're just going to play around with it a little, run the odd game, watch demos and maybe use some classic AGA software (PPaint etc.) then I see no reason why 3.0 should be insufficient.
