Your tank is too small already for 3 turtles. You will be needing a huge tank if you plan on keeping all three together, not to mention when they get older, aggression issues arise, and you have to get seperate setups.
Have you read through the whole habitat section on the main site?
http://redearslider.com/index_habitat.html
Fill up that tank as far as you can now, they need all the water they can get.
Do you have a basking area? UVB and heat lamps? Water heater? etc.
Anyway on to the filter. The Zoomed 501 is a piece of crap. It's not going to have the power you'll need. You want to cycle the tank 5x per hour or more. For now if your in need of a filter, you should get the whisper 40i.