Carrida - Factory brakes on a 74 Vette should have one bleed screw per caliper. In 1978 the rear calipers had a second bleed screw added. It is possible that the calipers on your rear are actually replacements for a newer model year. The front should only have one bleed screw per caliper, regardless of year. When bleeding the calipers with two bleed screws, be sure to bleed the inner then outer on each caliper.
1979 Toy - You say you have three in the rear, do you mean three per caliper? I'm confused:confused, I've never seen such a setup. Do your calipers have a manufacturer casting on them, if so who is it? Maybe someone else has seen such a thing. As for two up front, if you mean per caliper, again I've only seen the single bleed screw design up front.
Actually, it looks like three bleeders. The rear have two bleeders up top pointing to the front of the car, and a plug in the rear. At first glance it looks like a third bleeder. I guess you could always unscrew it and make it bleed.