A, B, and C are the gates. It makes no difference (IMO) which gate you line up at. I suppose if you want to be right in the middle you'd want to be in Gate B, but I don't really see any advantage to doing so. The rows you line up in are 1, 2 and 3, and as PP said, if you don't want to have legs dangling above you, you want to be in row 1. You could ask to be directed to row 1, but if row 1 is already full when they get to you, you'll have to wait until the next flight.