Ubuntu

It's not possible to get it all in one installation...

I'm not sure what you mean by "one installation". But you can certainly install all three OS's on a single system. I have a system with XP, Redhat, Fedora, Suse, Debian, and Scientific on it. And I hope to add several versions of BSD and at least one version of Solaris to that.

Yeah I know. I have Windows2000, and mandriva linux.

It's just you would have to do alot of partitionning, and that might requier about 200 GB
for good hard disk space on all of those OS'es

Not true you would not need anything like 200G and three Linux flavors could run in as few as four partitions. I have a triple-boot Ubuntu/XP/Solaris on a 60G disk on my laptop and I have plenty of space in all of them.

Orait..as promised, progress on the installation. I've done it in 2 methods:

1. As per corona688 suggestion :

/dev/hda1 -- 200mb of ext2 to hold kernels and grub settings.
/dev/hda2 -- 512mb, swap space. They can all use the same swap.
/dev/hda3 -- A couple gigs of ext3 for common space. Mount it on something like /opt/shared for all three distros, so you can dump a file there in one distro, reboot into another distro, and still have that file.
/dev/hda4 -- All remaining space, as an extended partition.
/dev/hda5 -- First partition in extended partition. Holds root partition for RedHat
/dev/hda6 -- Second partition in extended partition. Holds root partition for Fedora
/dev/hda7 -- Third partition in extended partition. Holds root partition for CentOS

1st installation of RedHat is succesful, but when it comes to 2nd installation of Fedora, it fails. It seems that they doesnt support the /, /usr/, /var etc to be cross platform. So, i when for 2 method.

2nd method:

I've separated the 80GB to 3 partition. The 1st installation of RedHat on the first partition with : /, usr, var, tmp, opt, swap, etc. was succesful.

2nd installation of Fedora i did the same spec as per RedHat except, i've left out the swap space and intended to cross platform the swap. So, when i click ok, it requested to format the swap and i proceed. after succes installation it rebooted and i notice that on the grub, the RedHat is gone.

Is there some steps missing here? coz when i redo the RedHat installation it goes vice versa.

Please help me. Thank you in advance.

It's probably replacing your grub.conf instead of adding to it, so it forgets how to boot the previous OS.

You'll need to keep a copy of your grub.conf before installation, and re-add the relevant settings when something inevitably overwrites it, or worse, changes what partition grub looks for grub.conf in! That's why it's best to have a seperate /boot/, easier to keep track of what's booting from what, where.

Also, I don't understand your statement: Are you trying to make them all use the same /usr/, /var/, and so forth? Nono, give them their own!