pictus cats grow around 5+ inches and are very very very active fish, swimming both horizontaly and vertically. me, myself i would try to give them as much room as possable. I have looked at them for a while now and decided that with my tanks being planted and having little open swimming space (driftwood and slate caves and rocks in my tanks) that they wouldnt be too happy in my tanks.
so me, i wouldnt house 3 of them in a 20 something with a bigger footprint would be better suited for them but thats just me
20g just isnt big enough for these little guys. they are seriously one of the most active fish i've ever seen, and i wouldnt really want to house them in anything less than a 55g tbh.
If they're all supplies you would use and they're in decent condition, it's not a bad deal. Assuming "supplies" includes gravel/sand, filter, heater, light/hood, and maybe decorations.
$150 for a 55gal tank + stand is a great deal anyway reguardless of equipment around here anyway. For a 55gal new combo its gonna run you $300-350 where i am. I'd jump all over that deal. Assuming it holds water.