dwarf painted turtle

Question
This last spring we found some turtle hatchlings in the road infront of our yard. We picked up 3 of them before they got run over and put them in our yard pond. Over the summer we fed them well. Two of them almost doubled in size but the third has not grown at all. He is about the size of my thumb. He is very active, seems very healthy, and eats well. However, he has not grown. He was noticably smaller when we found him. I am just wondering if you had any explanation for his size? We named him Tralfaz and we are rather curious.
Thank you.  

Answer
Hello how big is the pond that they are in?  I wonder if  there is an issue of territory. Can you remove him to feed seperatly?
Some times the runt (if you will) usually do not make it there for congradulations on that already.
They do not all grow at the same rate either,
Let me know how it goes! Id try seperating them for feeding,