the first sentence, (which is to be discussed) is a clause, it can take the place of an appositive. for example you want to explain s.th. you say:
1-This mathematic exercise, which is to be discussed, is very hard to solve.
in this case, (which is to be discussed) is used to give more informtion about the topic you are using.in this example, the doer is not forced and made to do s.th but the sentence is just giving some information.in this sentence, there is a plan for doing some tasks.
2-The mathematic exercise,which has to be discussed, is very hard to solve.
in this case, (which is to be discussed) is used for the same reason. but there is a force in it. that means that someone must and has to and is forced to do s.th .
3-in my opinion, the second example doesn't make sense, nut i may be wrong, and it may be meaningfull. but i didn't find any situation that this example is used in it.
hope that helps