I had to create 2 logical drives (LUNs) from the usable 3 TB RAID 5 array because the disk where Windows is installed must be a MBR disk, which is limited to 2 TB. By having 2 LUNs (100 GB for the OS and the rest for storage) Windows sees them as separate disks, so you can configure the bigger volume as a GPT disk, bypassing the 2 TB limit.
When you press F8 at boot time to access the P212 setup program, you do not have the option to create logical drives inside your RAID array. To enable this function, you must boot on the provided "Easy Set-Up" CD, and access the ACU there.
After that you install the OS on the smaller volume. When Windows is installed, you can convert the second logical drive as a GPT disk, with full access to the disk space.
Thanks to gregersenj from HP ITRC forums for pointing me in the right direction !
See his post at http://forums11.itrc.hp.com/service/forums/questionanswer.do?threadId=1420815