What is the adavantage of a 3 bladded prop over a 2 bladded prop on a trolling motor?

You could say that the advantage could be more forward thrust if the pitch is the same of a 2 blade prop.  But you will create more drag and use more battery doing so.  Also, the 3 bladed props aren't real good at being weedless.  I know my trolling motor only recommends a 2 blade prop and both are weedless if I stick with MinnKota's charts.

With a motor guide a 2 blade prop is considered a power prop,and is faster. A 3 blade prop is  better for weeds and is not as noisy. Don't have much experience with Minn kota.........Al

The less blades the more efficient the forward thrust. A 2 blade prop will push the boat the fastest in open water, but the longer blades get wound in grass quicker and also lose some torque to break free due to the mechanical advantage the weeds hold on the TM because of the leverage they gain on the longer blades. A 4 blade TM prop is pretty darn close to unstoppable in the grass, however it struggles to push a boat in open water with much speed at all because they have short stubby little blades. A 3 blade TM prop is a compromise between the two. You don't lose a lot of speed/pushing power, but they're better in the grass than a 2 blade.