93 Camaro Overheating

My 93 3.4L camaro is overheating i have replaced many things including coolant fan relay, radiator cap, coolant temp. sensor....i dont know what the problem could be...it will overheat within 3 minutes of me driving it, and there is steam coming out of the oil fill cap...cant be good news...i was thinkin a very serious problem...does anybody know what else it could possible be?