A couple of things to consider 1) if you were sucking coolant into the combustion chamber you would likely notice the exhaust color change to white and would likely see some tell tale moisture as the exhaust stack. 2) Number 1) would not put coolant in the belly pan. Have you checked all the hoses, any pin hole leaks, everything tight in the cooling system, did you check the radiator cap? Are you running a mixture of anti-freeze and water or straight water. If straight water or weak mix are you boiling the water and overflowing the cap? Anyway, my bets would be on the simple things and would check that radiator cap. Last is 3psi a reasonable number...not sure.
Hope this helped and good luck.