installation 3 OS in one comp?

[problemchild]

The problem is that lilo and grub both live in the master boot record of the hard drive, and they keep overwriting each other. Red Hat and Mandrake will not autodetect each other like they do Windows, so each bootloader has to be configured by hand. I recommend using grub, as it's a little easier to deal with in a situation like this. You can boot either one of them for this.

You need to understand how grub identifies partitions. Grub counts both disks and partitions starting from 0, but Linux counts from 1. So the first disk is hd0 and the first partition is hd0,0. Extended partitions start at 5 in Linux and 4 in grub. I will assume that you have Windows on hda1 Red Hat on hda5, 6,& 7, and Mandrake on hda8, 9, & 10. Enter the following commands:

grub
root (hd0,7) [wherever your current root partition is]
setup (hd0)
quit

Then edit your /boot/grub/menu.lst like so. The entry for whichever LInux you're in and Windows should already be correct. Just add whichever third section you need:

default 0
timeout 30

title=Mandrake Linux
root (hd0,7)
kernel /boot/vmlinuz root=/dev/hda8 initrd=/boot/initrd.img

title=Red Hat Linux
root (hd0,4)
kernel /boot/vmlinuz root=/dev/hda8 initrd=/boot/initrd-2.4.18-3.img

title=Windows
root (hd0,0)
chainloader +1

[EDIT] I just took another look at your partitioning scheme,and it looks like Red Hat is on hda5-11, so Mandrake probably starts on hda12. Adjust accordingly.

[doktorf00bar]

Good answer problem, that never even occured to me....

[ac1dsp3ctrum]

Hmmm, You might have a few problems with each distros swap space (/swap).....

[problemchild]

I don't think so. Red Hat and Mandrake will both autodetect the presence of a swap partition and use it. They will share one swap very nicely by default.