Suitable work surface and tolerances for casting concrete countertops
I could suggest that maybe you would want to carefully select your 3/4" plywood first to make sure the sheets are very flat to begin with. Then I would select multiple sheets of this flat material and glue then together to achieve a 1 1/2" or 2 1/4" thickness slab. This will give you the stiffness that you want.

The fact of the matter is that a single sheet of even 3/4" plywood (especially the lower cost sheathing grades) flex more than you think. A higher quality plywood with 8, 10 or 12 layers would be stiffer but even then it may still flex too much if just one sheet was used. 

Stiffening with separate lumber like 2x4's may work but 2x6's on edge would be much better. However the stiffening would need to be made into a frame or matrix across the bottom so that the above plywood cannot flex between the stiffening members. Building that is possible but likely takes very straight lumber that is constructed into the frame using tolerances as good or better than you used for the molds.