The number of spark plugs has nothing to do with whether it's a 2 or 4 stroke. All we know is it's a 3 cylinder. Do you have to put oil in the crank case? or mix it with the fuel (or have oil injection). Better yet, if this is an older motor, it's probably a 2 stroke.
If it's a 3 cylinder and got 3 carbs >> it's a 2-stroke . Like this one ??
If it's a 3 cylinder and got injection ( DFI, Optimax ) >> it's a 2 stroke .
If it's a 4 cylinder and got injection ( plain EFI ) >> it's a 4-stroke .
