I think you'd be stretching it to mount them 12" off the tank and cover 24" x 24" with each fixture.
60" - ( 24" + 24") = 12"
That leaves an unlit foot of tank...which might look something like this.
Plus, as you can see in the diagram, a tank only 18" wide kinda complicates this. With 80-90º lenses you can only raise them about 9" off the water before you start lighting up the room instead of the tank.
9" gives them around 18" x 18" of coverage. So you're restricted (for the sake of discussion) to 9" off the water.
60" - (18" + 18") = 24"
That leaves you with two unlit feet of tank.
Three units would give you better spread options at the more-ideal height, if it matters.
It might look something like this...diagram for a 180, but it's about the same as yours at 9"...notice there's no degree of light outside the perimeter of the tank....and as you can see from the side-view, coverage is still complete....there would be a few shady zones in the top layer of water:
Four would make coverage almost perfect, eliminating the shady zones in the top layer.
Four would also give you increased range of intensity...good for more demanding critters, e.g. clams, and good for allowing you to run each fixture at a lower % to extend emitter life. (Make sure you get some PAR or lux readings before you try clams....they like a lot more light than corals, generally speaking.)
