As above, really.
If you haven't already bought this 7 litre tank, I'd look at something a little bigger. It won't cost you much more to buy, it'll be easier to maintain, kinder to your fish, and will still sit on a desktop or a bookshelf (my betta lives in a 22 litre cube on a sturdy side table next to my sofa). You *can* do a betta in a 3 gal tank; personally I'd be more comfortable with 5 gal plus, like the other posters - they're often more active than you'd think, and they're very intelligent! If you like colour you could also consider endlers or maybe guppies (not in the same tank as the betta though!) - there's an excellent pinned thread with plenty of nano tank recommendations.
If you have the tiny tank already and are set on keeping it, you could set it up as a living work of art, with nano plants etc. I have a coworker who has a small planted tank on his desktop (no critters) and it's very effective. It's too small for fish IMO; I've never kept shrimp or snails, so can't comment on inverts.
Good luck.