I know this is an old topic, but I was just trying to figure out the answer to this problem.
There are three (3) 11x2.5 o-rings (91303-035-000) on the crankcase, because two of them go around the two center-rear cylinder studs.
The third one is the seal for the oil path plug cap on the left side of the crankcase, near the base of the #1 cylinder.
Incidentally, there are two more o-rings just like them, on the same two cylinder studs, but they go on top of the cylinder block.
(For some reason, my "complete" gasket set only had two, when I needed at least four, maybe five if I were replacing all of them. To make it even more confusing, Honda used four different part numbers for this o-ring.)