Hmm... I hope somebody else can chime in here. I'm wondering if they're fighting or... is her shell rubbing against her neck/head? I've never seen it that bad, but Tomi's shell used to rub on his skin and he was getting a bit of a raw patch. It went away though. Either way, the tank is still pretty small for 3 sliders, you should really keep an eye out for any aggression in which case you need to separate them. A quick solution would be to get one of those rubbermaid stock tanks, it's cheaper than another glass tank.
I don't know a lot about treating wounds so somebody else needs to answer this. Do make sure your water is clean to prevent any infection.
About your basking area, it looks very good, it might take them a while to get used to it so don't worry too much. You should leave pieces of their favorite food up there... start on the ramp so they see/smell it then put some pieces all the way on top. That will entice them to go up. You will know whether or not they used it when the food is gone. Also make sure it's warm enough up there so they'll want to bask. I have a question about your lamps... what are they sitting on? It almost looks like plexi. UVB won't penetrate plastic/glass so they won't get the benefits of UVB that way. You can replace the plexi with eggcrate/light diffuser or remove the plexi and suspend the bulbs.... or you can make an opening in the plexi so the light is unobstructed.
Also, is that gravel in your tank? If so, you should remove it. See this thread:
viewtopic.php?f=5&t=12532Lastly, you don't need to use VitaShell, they just want your money
