Northforker is right about a hundred different answers. I know people that swear for and against each arrowhead mentioned by Northforker.
But, back to your question which was 3 verses 4 bladed fixed.
The answer is not quite so easy. Any 3 or 4 blade broadhead that is straight can be tuned to fly very well. But the larger the diameter the more wind it catches. For any given weight, a 4 blade broadhead usually has less effective diameter than the 3 blade in the same broadhead. So, off the cuff, the answer is 4 bladed broadheads fly better than 3 ----- but not so fast.
Remember that I said that a straight broadhead can be tuned to fly very well. That is true, and because greater diameter means more cut veins and arteries, and therefore more blood trail, one should not necessarly discount 3 bladed boradheads.
Personally, I find no difference in flight between 3 and 4 bladed broadheads within the same weight and the same brand.