You would want to keep the babies separate like Kelly said. Not just in case the larger one picks on the smaller ones, but also the larger one can harm/kill the small ones unintentionally due to the fact that it is so much larger. Keeping the babies in their own set-up until they are much closer in size to your older turtle is a good idea. But even then some turtles just do not like companionship, so be prepared to have individual habitats in the future not just for the babies and the older one, but possibly for all 3 should the babies grow not to be compatible with each other.
You have as much chance of getting salmonella from your turtles (regardless of their size) as you do from handling raw chicken while cooking. Good hygiene and common sense like not letting them run loose through the house and washing after handling your turtle or their equipment is all it takes to remain healthy.
