I'm trying answer the following question. Yes, it is a homework question, so I'm not asking for the answer directly but would like to get some pointers on how to solve it.
So far I have spent several hours thinking about this to no avail. I know I'm missing something incredibly simple. I have tried various permutations of fixing messages m1 or m2 or m3 to be constant while others change and seeing how that affects the value of c3 when it is returned, as well as various permutations of fixing the messages to be 0^blen in order to try to find a collision. Hoping to get some pointers in the right direction.