Welcome.
This page has just about all you need to know about food:
http://www.redearslider.com/index_nutrition.html
Then, browse through the rest of the site to get lots of information on your new turtles.
Now, if it's possible, I would recommend only getting 1 new turtle, since RES are notoriously aggressive, and when they are full grown you're looking at about 100 gallons of water each. But if they are rescues, just be prepared to eventually have a huge tank, or, more likely, 3 separate setups.
Good luck!