My painted turtle is being bullied

My partner’s son has three painted turtles that have all lived in the same tank since they were tiny. One turtle is clearly the odd one out — he’s smaller than the other two, tries to keep his distance, and it looks like one of the bigger turtles might have bitten his tail off. Any chance he gets, he tries to flee from the enclosure.

What are the best options moving forward? Trying to rehome the little guy? Can painted turtles be released into the wilderness? Forgive me, I’m no expert here! Asking on behalf of the family.