I have 3 SAN disks of 1 TB each. Is it more efficient to make 1 large Volume group with multiple LV's or 3 Volume Groups with 1 LV each. The SAN disk is where all my backups will be dumped.
Speed is influenced mainly by the number of physical disks used and the SAN storage's RAID level. Type of VG influences speed too but as backups consists often of large archives this probably won't make a big difference.
Splitting the storage in several smaller VG could be usefull in case you need to backup/restore those VG themselves but as you said the storage is meant as sort of dump site for other backups you could neglect this aspect.
With SAN disks of 1TB size each I'd use one Standard VG with multiple LVs. If the SAN disks are on the same physical disks create your LV/FS regardless of the LV's allocation policy. If the disks come from different SAN-VG/Ranks/whatever (i.e. different phys. disks), create the LV with an inter policy of maximum. (Always try to use all physical disks, avoid creating hotspots.)