I was wondering if i could possibly squeeze some 33x12.50x15 tires on a 3" body lift, or should i play it safe with 31" or 32" on my 4x4 94' S10 blazer V6?  

i dont know what the biggest tire is you can fit under that with a 3" lift. But i always go about 2" smaller so you give yourself room to flex especially if you are going to do offroading. I would deffinatly play it safe and go a little smaller because you dont want to rub brand new tires.  

Also does anyone know if Add-A-Leaf kits are any good?  

Add-A-Leaf kits are fine, the ride will be stiffer, but they work.

If you could I would steer away from a body lift and lean more towards a full suspension lift. It's soo much nicer.

But to answer your question, 33's should fit no problem with a 3" body lift.  

Ya 33's should fit just fine under there witha 3 inch body lift.  I put 33's under my 99 z71 with out any lift at all (and i know that it is a bigger truck, but with 3 inches of lift you ought to clear everything just fine) If you are looking at lifting the back suspension, yes leaf springs will do the trick.  However, this can create a stiffer ride.  You can probly get away with shackles or blocks.  My budy put 3" shackles under his s-10 truck and it worked out fine after he cranked up his torsion bars.  But be careful about cranking up th torsion bars because i have been informed that this will put stress on your CV joints.