Thanks, sadly there is no room and no place for a filter to go on. I will add some food to help the bacteria though. I'm just wondering if I should change the water after 24 hours in case there might be anything harmful in the tank. All I have going in there are a couple o rams horn snails and 2 male Endlers.
Ideally, you should fishlessly cycle your tank before adding any livestock. Have a read of the beginners section - there're some great articles on there. Cycling your tank means that enough good bacteria is in your tank to deal with fish waste and turn it from highly toxic substances (ammonia --> nitrTte) to less toxic (--> nitrAte). It can be a lengthy process and take anywhere from a few weeks to a month and is done by adding ammonia to the tank, which represents the waste from fish (household ammonia seems to be the most common method). Fishless cycling means that no fish have to endure toxic water conditions during cycling - surely it can't be pleasant even for the hardier species.
If you have other tanks which are established and cycled, I would add media from those tanks into this one if possible. If you manage to mostly fill the media compartment in the filter, I'd say you could add fish/livestock pretty soon. If you completely fill it, feel free to go and add fish. Don't take too much from your existing tanks though - I'd put some of the new media from this tank into the other tanks to replace any you take if you do this. If you're not able to donate a significant amount, don't worry - this will just speed up the cycle

If you're not able to add fish for a few days or even until tomorrow, I might be tempted to add a little fish food or a small amount of ammonia into the tank to keep the bacteria 'fed' until you do.
In a 3 gallon you're not looking at much stocking-wise, though. I wouldn't add more than a single male betta in there, possibly with some shrimp or a snail (if the betta gets along with them, sometimes they can see shrimp as snacks and peck at snails, though sometimes they leave them completely alone, it's all down to the individual bettas personality). UGFs (Under Gravel Filters) like the one in your tank aren't that great in my experience though - might you be able to get a small internal one to go on the side of the tank instead?