Our transmission crossmembers for the 1982-1992 GM F-Body Camaro and Firebird have three (3) slotted mounting holes per side. The car itself will have three (3) mounting holes per side, but only two (2) of them are threaded.
You only need to use two mounting bolts per side. The reason there are three mounting holes is because the 1982-1983 cars used the front four set of holes and the 1984-1992 cars used the rear four set of holes.
You can tap the two holes that are not threaded from the factory and run three (3) mounting bolts per side, but it is not required.