I have actually heard a lot of this happening.. and it doesnt just happen between big and little turtles, it can happen between any turtles. But a 10 gallon is NOT very big for 3 turtles. The 'rule of thumb' around here is 10 GALLONS per INCH of turtle. So, even if all of your turtles shells were about an inch long, you would still need a 30 gallon tank!! so it sounds to me like they are probably crammed... and this can lead to territory disputes (because turtles are VERY territorial), and since the bigger one was well... bigger and stronger, he won! i am also not saying that getting a bigger tank will help because sometimes turtles just do NOT get along with each other, and they need to be separated! but i dont see what getting a larger tank could hurt. Also, you should probably have something for the baby turtle to hide in... incase the big turtle does decide to attack again! Odds are they should already have been in a larger tank anyways!
Turtles should grow about an inch per year. And about the feeding thing... i hope you are NOT just feeding the turtles pellets. During the first year of life (usually an inch or a little more) the turtles will eat mostly pellets (but should ONLY be fed enough that would fit into their heads)... but after this first year or so, they should ONLY be fed the pellets once every 2 or 3 days, and greens (such as red leaf lettuce, dandelion greens and even collard greens) can be suplemented daily... you can feed the turtles as many greens as they will eat. But the problem with feeding them as much protein that they want is that they will become overweight... which then not only leads to shell deformities but also many health problems and would probably result in an early death.
i hope this helps...
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~ Evie 
Pets:
RIP 0.1.0 German Shepherd {Dusty}
0.1.0 German Shepherd/Collie mix {Shadoe}
0.0.2 RES turtles {Booger & Snot}
1.0.0 African Clawed Frog {Bingo}
0.0.2 Snail {Gary & Larry}