Did you get all Bettas at the same time? Did you tell the person who sold them to you what tank they will be in? A 3 gallon is not a medium sized tank, it is a very small tank. Do you think it may be possible for the store to take 2 of them back? They may give you credit towards the new tank you are planning on getting, or some supplies!
You will find that until June/July you may not have any Bettas left, but if you just keep one you may have a chance. A Betta really needs at least 5 gallons to be able to swim a little bit. It should have a filter so you don't have to worry about toxins building up in the tank. Have you heard about cycling a tank? It's not that hard and once it is done the tank will be a whole lot easier to maintain. The tank should also have a heater because it is a warm water fish.
If you want to keep many Bettas, then remember they got their common name "siamese fighting fish" for a reason. All males absolutely have to be in their separate tanks. There are sorority tanks for female Bettas, I think the ideal number is 5 or 6 females, and I believe the tank should be at least 20 - 30 gallons (someone please correct me if I'm wrong).