Here's the brutal truth: there isn't much you can do with a 3-1/2 gallon tank - and I cringe to even call that a "tank" - other than a betta. You have to temper your expectations. Those tiny tanks are manufactured because they *sell*, not because they are good places for fish to live. You certainly can't keep free-swimming fish like barbs or danios in it. Besides the space issue, those are also tropical fish, and won't do well in an unheated tank (which I assume it isn't).
An ADF (dwarf frog, NOT clawed) or two would work. A couple of the small aquatic crabs maybe. Some ghost shrimp. *Maybe* three dwarf cories, but I hesitate to encourage even that. No matter what you put in there you'll have to commit to doing sufficient water changes. I wouldn't recommend barbs or tetras for a tank that small. There's just not enough movement space. Also, remember tropical fish and unheated tanks don't mix.
Regarding returning the fish you have, few LFS's will turn down free fish if you're just looking to free up your tank for something else - but they generally won't refund your money after a few days. Just ask the LFS if they'll take them off your hands as a donation, and lump it. A lot of folks do this when they want to change the fish in a tank. If they won't take the fish because they're sick or diseased, you either need to treat the fish until they're well, treat the fish until they die, or euthanize them (and this only as a last resort if you're certain they'll die).
If you aren't willing to work with a betta, is there no chance you could put together an inexpensive, heated 10-gallon setup? Your choices are still somewhat limited in a 10-gallon, but it's a whole world to work with more than what you have currently.
Hope that helps.
pendragon!
