2 per head will be the disc type (the hole will have a step so the plug can only go in so far).
The third head plug will be one of the following:
– Disc style plug like the other 2 (mostly ‘68-’69 engines).
– The press in cup style (like a standard freeze plug) (some ‘68-’69 engines and 425 / 368 heads).
The 425 and most 368 heads will have one plug and one press in 90º heater outlet (no longer available) from the factory.
– Threaded head plugs (pipe thread) – usually one plug and one straight heater outlet (most ’70-’76 engines)