It's 4 for the price of 3. (3*33% discount on 3 full-price titles = 100% discount on a 4th title) Which is... kind of a weird thing to advertise, not THAT good. 33% off everything sounds better, imho.
If you buy 2 games @ full price (assume $60), that's $120.
If you pick 3 games, which drops the price of each to $40, that's also $120.
3 for the price for 2, no? It's late so my math may be off.