Why a 3 port Throttle Body?
 

Why did mazda use a 3 port throttle body on rx7's? Why not just one port like most cars.

As lame as the question may seem I am just curious.

The secondary throttle bodies open up different for cold temp. That way you don't stomp on the throttle and cause a sudden rush of air.

It helps with drivability. Under low throttle angles, only the primary throttle plate is open so air is only going through the primary ports. This keeps the air velocity through the port sufficiently high so that you do not have issues like surging/running on 1 rotor/etc.

you are thinking of the double throttle system (which are beyond the main throttle plates in the air stream), which isn't what he's referring to. even if you remove those there are still 3 butterfly valves.

and yes, the big one goes to the 2 primary ports. If you rotate the throttle linkage you can see when the secondary butterflys start to open. The point at which they start opening is actually adjustable... not that you really would want to do that.