Hello. Welcome to the site!
Like Mumma said, there are not too many options for a 3g tank. Sorry to tell you that tiger barbs won't work out in there long term.
A couple options I suggest you look into are a single betta fish (there are more species of betta besides the bettas you always see in cups), or a couple of least killifish. MAYBE a pygmy sunfish pair. A betta is going to be the easiest fish to get though, the others you would likely have to order online. 3g is on the smaller side of acceptable bettas tanks IMO
, so I suggest a female betta. There are some awesome colorful ones out there though. They have really fun personalities.