There are millions or billions of quadrupeds surviving on 3 legs every day. 4 legs is evolutionarily superior to 3 but 3 gets the job done, especially if you don't have to worry about agility or repair.
If the robot has two legs with wheels at the ends it could combine speed and flexibility. Adding a third leg for additional stability when the robot needs it might be great. But the third leg would probably need to be between the first two and I don't think humans are mature enough to have that.