if You have three settings,do you have C.V.carbs?,like some Yamaha's,ones with flat domes on top?,if you do,then I'm guessing one of your fuel settings is marked .P.R.I,meaning prime.that basically dumps neat petrol into the carbs as a starting aid.the choke works as a enriching,when the engine is cold,if you open the choke fully,then more air than petrol,passes into the carbs,which in turn,makes the mixture richer,which in turn,makes it easier to start from cold.obviously,when the engine is warm,leave the choke off,or alone,you won't need it,