To update the backplane firmware, make sure you have the IPMB cable connected correctly.
Cable: Use the 4-pin IPMB cable provided with a hot-swap drive enclosure.
Backplane Connector: Use the white 4-pin IPMB connector on the backplane.
Mainboard Connectors: Use the white 4-pin HSBP connector on the server board.
The 3 DIMM configuration seems a little strange to me... Since the system has two memory channels you may want to use 2 or 4 DIMMs to get better memory bandwidth.
There are two documents available that talks about the cabling guide for the RAID controller/Chassis/backplane being used in your situation. You may find it helpful.