I have a driveway that is cracked and is going to be replaced with new concrete. Since there are large trucks making turns on my driveway, I have asked the contractor to install 3/4' rebars. He will install it 3' x 3'. Will this be enough or should it be closer.

This is one evaluation:
Rebar Installation & Sizes (http://www.renovation-headquarters.com/rebar.htm)
I think that 3' spacing is too much.
Thickness of concrete and location of resteel (below midpoint of thickness) and clearance from bottom are also important.

3/4 rebar is ovrkill, 3' spacing is too large to make a difference and prevent cracking, if you have large trucks turning on your driveway you need a thicker compacted base under your concrete and thicker concrete, a standard 3" slab with a 4" base will not support heavy trucks no matter how much or what size rebar you have in the concrete. I would recommend a 6" slab on a 8" compacted base