Why 3G Consume More Battery than Wifi?
First of all, all wireless systems use the most power when transmitting, ie actively powering the antenna to upload data.

Now, just think of the ranges the different systems have to bridge. WiFi works well for like 30 meters, or 100 meters outdoors. Usually, the nearest cell tower is considerably farther away than those 100 meters, therefore, 3G needs more power than WiFi.

Add to this that 3G is capable of opening multiple connections at once, and the fact that 3G autmatically boosts power output on weak signals, 3G needs more power.