Why do BLDC ESCs have more than 3 mosfets?
In a half bridge you need two mosfets, a high side switch and a low side switch to reverse the current. The reason for a full bridge is so you can reverse the current (and voltage polarity) on a load. 

The diagram below shows a whole bridge (two half bridges) and why you need two switches.  

This can be extended to three half bridges in a bldc motor since one half bridge is on, one half bridge on but on the low side (is sinking current) and the third bridge is off. These are constantly switching but this gives you the general idea.







simulate this circuit &ndash; Schematic created using CircuitLab