So I am trying to reverse the polarity on my brushless motor. It has three wires so I am thinking that I should just switch the black and red while leaving the yellow alone. Is this correct?

If you mean reverse the rotation then yes, switch any two wires and it will run reverse.

Why do you want to reverse polarity/ Is motor running in wrong dirction/ If that's the case then switching any 2 of the 3 wires will change it's direction. That doesn't reverse polarity but does do what you want .