With out a plenum, its not going to matter which cylinders you choke. As long as one has a fast idle and a choke it should start. Using the center carb is easer to set up since the exhaust heat plate is close. A lot of multi carb setups run the center as primary and the ends as a secondary, thus the center choke. With multi carbs and one large plenum where all three run as one, it will not matter. You are going to be faced with more of a challenge getting three sets of cylinders to run equal.
In your case, you have to be 100% sure all three YH's are the exact same year and model number. With out the plenum to mix all the vapors together, you have to be right on with each mixture.
A flow meter like the Uni-sinc (
http://www.larryscorvair.com/Merchant2/m...egory_Code=LCTO ) will be needed to dial in each carb. Do a e-bay search for carburetor tools or balance tool or what ever you can think of. There are some nice tools used for multi-carb motorcycles that should work really well.
Jetting will be just a guess till you run it a bit, it won't take long to figure out if it rich or lean. Start with what came stock in the carbs or one leaner.
As far as vacuum goes, a properly set up multi carb unit will have just as much vacuum as a single carb set up if both are sized correctly.
Joe
