like ralph said thats way too small for just one of those guys much less all three unless you plan on culturing live food and feeding 3-5 times a day. but if you're set on putting fish in that tank look into smaller fish. heres a few that would be the right size to be kept in picos some of them i have kept before.
green banded goby, catalina goby (coldwater/temprate so be carefull with them), barnacle blenny, clown gobies, red banded goby, citron goby, yasha goby, flaming prawn goby and there a few others. and you can also fill the tank with coral. there are also lots of inverts that you could keep in there to add some life to the tank such as sexy shrimp as mentioned, bumble bee shrimp, pistol shrimp (you could pair one with a goby), scarlet hermit crabs (very pretty), emerald crab, ruby crab, porcelain crab, acro crab, etc.
and don't take this the wrong way but i can tell you're fairly new to the hobby so please make sure you research any animal before you get it specially if you plan on keeping it in the 4 gallon tank you have. if you would like any help setting it up let me know ive done a few picos before.