How nice of you to rescue them and give them a larger home
Besides a basking dock do you have the other turtle necessities like proper lighting, a strong filter, and a submersible heater? Once you have the basics covered you can add things to liven up the tank such as fake and real plants, hatchlings especially enjoy resting on floating plants.
Growing that much in such a short time is not healthy... it can put too much stress on the organs, and I can't imagine it's healthy for the shell. Take a look at the nutrition section to get ideas on what is ok to feed turtles, and how often. If they're babies you should pellets everyday- the amount that would fit into their heads. If they are 4inches + you should feed them every other day. As well, you can start feeding the veggies everyday.