You've got a tank, that's one hurdle you won't have to jump over.....
You didn't say anything about a filter. If you need one, go big, especially with three turts in one habitat. Things can get messy....I use fluvals, but for a beginner, I suggest a rena filstar xp3 canister. Easier to set up and maintain. You're gonna need a heater, at least a 250 watt..300 would be better, if you can find one. Tronic, Stealth, Ebo-Jagger are all good choices. Also get the water level up to at least 3/4 full, more if you have a basking area that will allow it. And a screen top (turts are master escape artists).
Lighting: Heat lamp first, once the basking area is set, get a dome type lamp with a regular 60-75 watt (depending on distance) light bulb. Some keepers just place the fixture on top of the screen cover, others use a clamp and attach it to the tank. You'll also need a UVB light, Zoo-Med 5.0 or 10.0, and a fixture to go with it. Both lights should be arranged so they are focused on the basking area. The heat lamp needs to be positioned so the temp on the basking area reads around 90F. The UVB light should be placed 10-12" from the basking area. (If you go with a 10.0, no closer than 10 inches) Get a timer and run them 12 on, 12 off (day/night cycle).
Most of this stuff can be had @ Big Al's
http://www.bigalsonline.ca/
Welcome to the Forum and the wacky world of turtle keeping! Feel free to ask as many questions as you like.
Now if I could just have a few minutes with the person who abandoned those turts.......