This is how to properly synchronize your ITB intake. First up you will need to buy or borrow one of these….
There are several different designs but they all do the same thing, they accurately measure the flow of air into the velocity stack. Using a vacuum type of gauge or set is NOT as good and the results will reflect this.
Firstly make sure each balance coupling has a decent amount of pre-load this will help your itb to close fully at idle and allow adequate adjustment in both directions when you synchronize. Now try to get all the butterflies to close at exactly the same time as best you can, open the idle screw all the way out and loosen the balance couplings on the shafts. Ensure all butterflies in each body are all tightly closed before tightening the coupling clamps again.
Adjust the idle screw to open them just a little and start the engine. Set the idle slightly high 1200-1500 rpm is fine just so that the engine runs smoothly enough not to hunt up and down. Then measure the flow into the body with the master linkage attached. Then measure the next one and adjust the coupling so the flow matches the first, the engine should get smoother and the flow into the first may change so go back and forth. Then move to the next one and adjust the coupling in the same way until the flow also matches the first two, once again the engine should get smoother.
Dab the throttle a couple times letting it snap closed to make sure they are all fully closing and recheck the flow on each one. If they are all even set your idle back to the required idle speed. Remember the better the synchronization the better and smoother your engine will run.