That's either plugged because there was a leak or it was a pool vacuum line. A picture of the equipment pad would tell us the whole story
I have 2 return jets on each side of the pool.. 4 total .. skimmer and main drain.How many other suction-side ports are in the pool? (Skimmers, main drain, other wall ports that pull water in.) You have two lines going to your pump so that's two suction lines but there's that weird third pipe that may have been plumbed at some point. I would guess the mystery port is either one of the liens to your pump (if you only have a skimmer and nothing else sucking water in) or it goes to that mystery line that's capped off.