The third SAS port is used to connect rear backplane. If you have only one PERC controller on your config then first 12 drive will be show under SAS connector 1 and last 12 drives will be shown under connector 2. If you have 2 PERC controller then first controller will see first 12 drives and second controller will see last 12 drives. Please note if you have 2 PERC controller, you can not create a RAID using drives between 2 RAID controller. All drives for the virtual disk should be from same controller.
Hello, we have 2 Dell PowerEdge T750 with the PERC H755 front and the BP15+ backplane.These each have 8x 1.6TB SAS SSDs and 10x 7.68TB SAS SSDs, I would like to RAID pair each and distribute them optimally in the slots.How is the BP15x internally wired to the RAID controller?With the old backplanes, the first 12 slots were on one RAID port and the other 12 slots were on the 2nd RAID port. I.e. I distributed the disks of each RAID 1 between the phsical ports so for the 1st RAID one disk in slot 0, the other in slot 12.How is it with the BP15+, since this is according to the user manual only with one connector on the backplane. Greetings, DellUser
[PERC H755 with BP15+] Optimal disk distribution across the RAID controller 
ports