Merc 50 hp 3 cylinders, but only 2 carbs

[fishin'musician]

I've got an '86 Merc 50 hp engine with 3 cylinders, but it has only 2 carbs. Does anyone have knowledge why this was built this way?
Which carb feeds two cylinders?

 

[Faztbullet]

The top/bottom carb feed middle cylinder

 

[racerone]

Top carburetor feeds #1 and say 1/2 the needs for #2----Bottom carburetor feeds #3 and other 1/2 for #2 cylinder