How to I calculate the AC amperage draw of a 12 volt 3 amp AC/DC transformer under full 3 amp load?
What PlasmaHH said is the truth. You can only figure that the power was loss-less
through the transformer. In other words since the power is (approximately) 3A X 12 VDC = 36 Watts I would estimate that the power on the primary would be the same
which is to say:

W = E X I (without the power factor) rearranged 
36W = 120 X I &lt;=> 36W/120 = .3 Amps which again is 36W without knowing the power factor, eddy losses etc. You would need to measure the power and phase angle on the primary to know the exact measurements.