Hi ilppp, welcome to NeoSmart Technologies!
Please don't cross-post your questions to several forums, how will we know where to look for a reply?
First, partition your system as follows:
Partition 1: 50 GB Primary Partition
Partition 2: 20 GB Primary Partition
Partition 3: 10 GB Primary Partition
Install XP first on the partition of your choice.
Install Vista to the partition of your choice.
Install OS X to the partition of your choice.
You must install them in that order.
Now from the new OS X bootloader, boot into Windows Vista. From Vista, install EasyBCD, and open the bootloader management page. Select "Reinstall Vista Bootloader" and then go to the "Add/Remove Entries" page, and add an OS X entry. Make sure Windows XP is listed in that list, else add it as drive C:.
Reboot to check, it should be working.