The reason for the recommended 2 foot minimum for the height is the substrate.
You said you were going to use 10-12 inches of dirt, which is very good. However, you will also need the standard stack of boards on top of the substrate under a heat lamp. With 18 inches of height, minus the 12 inches of substrate, that only leaves you with 6 inches total to the top of the cage. Allowing room for the monitors to lay on the top level under the heat lamp, you'll end up with only two boards stacked and the lizards basking within an inch of the lamp. Just not enough head room.
I would recommend 4x2x2 as the minimum cage size. I'd still use that size myself if I only had one ackie, but for a trio definitely.
Keep in mind that ackies are quite active monitors when the heat is correct, and they will use the space you give them. Trying to save on space with them is not a good idea, the more space the better.