Hi there,

I will try to explain this as best as I can though there might be some out there might have a different interpretation but I will try my best.

Represented by:  This could be used in the context of people, nations or organizations. So, for example: 'The United States is represented by Ambassador Charles Adams in Finland.'  

Represented In:  This can be used I think for ideas or ideologies.  For example, 'The ideology of Christianity is represented in The Bible.' 

Represented with:  This one seems a bit tricky.  You could maybe use this phrase in conjunction with an action perhaps, For example, 'My character is represented with my actions.'  Someone else might be better able to explain this one.

I hope this helps