"Can anyone please give me a rockhard how-to on how-to add SuSE and Mandrake to my Fedora installation"
Essentially what you do is install all three distributions to their own partitions. You can have a single swap partition shared by all three. At the end, one of the three distributions will be in control of the Master Boot Record. You boot into that distribution and change the bootloader configuration to triple boot all three distributions. The bootloader configurations of the other two distributions is irrelevent.
Here is a thread that discusses the problem:
If you have any further questions I suggest that you post them on the other thread.
Be prepared. Create a LifeBoat CD.