You can add multiple Units to one Volume Group (VG0) in the Unit Manager (use the pull down button. So if you have 3 different Units:
Unit S0 - VG0
Unit S1 - add to VG0 (Unit S0)
Unit S2 - add to VG0 (Unit S0)
After adding a new units you can define Logical Volume, swap, and space reserved for snapshot in iSCSI Volume Manager function. Here you will see the combined disk capacity in iSCSI Volume Manager.