if you look up the manual for the backplane, it will tell you. but basically, it is the 2 ports closest to the side edge. the 3rd one closer to the middle of the backplane should be for downstream daisy chaining to another backplane. that said, I wonder if it is possible to run 3 SAS cables and get 12 links @ 6Gbps? might be possible, i just haven't tried it...
The documentation on what port is for what is only a guideline, a best practice. In reality the expander chip is basically a switch and all ports are created equal (that’s an oversimplification and a white lie - but for practical purposes it is ‘correct enough’).
The LSI expander chip supports a maximum link size of 8. So connecting a third 8087 cable to the same HBA will still get 8 lanes of throughput, not 12.
You actually can connect two different HBAs even on the EL1 expander. Generally the other HBA would be on a different host. Managing it is tricky, however. They both can’t manage the disks at the same time. Normally this would be done for an active/standby configuration with fencing to ensure only one host was active. Unless you REALLY know what you are doing this is not recommended.
The EL2 backplane actually has two separate expander chips with the second one wired to the 2nd port on dual-ported SAS drives. When using SATA drives or single-ported SAS the extra expander and it’s ports are pretty much useless and just waste power.