I don't like to do less than a four inch slab. The thin spots (and there will be some) on a thinner slab will really kill you.
Some wire mesh will help reduce shrinkage cracking, but won't really strengthen the slab much. For that application fence wire is probably OK. Hook it up off the bottom as you pour, so it's actually embedded in the concrete. Try to get the wire in the center of the slab, as you want about two inches of cover on the steel.
Concrete shrinks when it sets. It cracks when it shrinks. The grooves you see in sidewalks and concrete parking aprons, are contraction joints to force the cracks to occur where you want them, and to hide the cracks. (If you can't prevent something, call it deliberate. /forums/images/graemlins/wink.gif ) The joints should be spaced d/24 maximum, or two feet per inch of slab thickness. Make the joints even and the panels as close to square as you can.