How to fill recess in fireplace before tiling
Yes, I agree with you. Use three layers of 1/2" Durock. Attach all 3 plies at the same time to the brick with 2-1/4" Phillips Tapcon concrete screws. 

Durock is certified as non-combustible tested to ASTM E136, “Standard Test Method for the Behavior of Materials in a Vertical Tube Furnace at 750°C” and therefore is acceptable in use as a fireplace facing.