Since Fleischmann does not make an ICE 3, I am quite certain that Roco coaches won't fit in any case.
If you've purchased the ICE-T, its tilting mechanism will prohibit that anything but the appropriate Fleischmann coaches can be used, apart from the fact that anything else would not be true to prototype.
If you've bought the ICE 2, the Fleischmann coaches are 1:93.something in length, while Roco's starter set coaches are 1:100. The combination'd look awful, apart from the fact that Roco only offers a 2nd class coach.