The rule of thumb is 10 gallons for each inch of turtle. 3 hatchlings (assuming they're 1 inch long each) would be a minimum of 30 gallons. You could get away with a 20 for a little bit if you needed to, but I think space is the least of your concerns. If you buy a 3rd turtle, you run the risk of them not getting along, or the new one bringing in a disease, or the smallest one being bullied and not being able to get enough to eat.
Best case scenario: new one is healthy, they get along great, you have to upgrade to a larger tank in a month or 2.
Worst case scenario: new res is sick, infects your other 2 with whatever illness he has, they all die -or- new res is fine, but he's a little pig and bullies the other two to the point that he injures them and/or they won't get enough to eat -or- they dont get along AT ALL and you end up with 3 full setups $$$$
If I were you, I'd wait. I also have 2 res and would love to get more but I would house them separately and I just don't have the money or the space right now.
Good luck with whatever you decide!

The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler