OSX 10.6 + OSX 10.7 + WIN XP?
You should do it in these order:


partition drive into two partitions — first for 10.7 and second for 10.6 and XP combined
install 10.7
install 10.6
run Bootcamp Assistant and install xp
— Choose 10.7 partition in Startup Disk in System Preferences


And if you only need 10.6 for Bootcamp assistant you may skip that because 10.7 has it. Once everything is set, use the Startup Disk control to write the Lion OS as the default for power on.