Real answer: how many animals can you think of with three legs?

Ah, so it’s impossible to make a robot unless it looks like an animal

Kangooroos?

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.

This one right here

How many animals have wheels? And yet wheels are a pretty practical choice for a lot of things.

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.

There is likely no benefit over 2 legs if you need to step over things. And if you don't, wheels are just fine. Maybe stairs changes this.

It's been done, I don't really know how the efficiency compares but I'm sure there's some research out there on it. The main issue I see though is that the advantage you get with 4+ legs is stability at slow speed - you can be stable on three legs while moving one, which makes precise movement easier.

With three legs, as soon as you pick one up you will start to fall over, so you either need legs with enough freedom of movement to shift the center of mass of the robot back to offset the lack of support, or some other way to shift the center of mass.

You have to balance with two legs, as well, but there isn't a transition from "stable" to "balancing" with every step - you're always actively balancing - which makes movement easier to plan.

Overall I suspect that tripedal locomotion isn't really any more efficient than bipedal movement, and it might even be less stable.

Just one example: https://spectrum.ieee.org/martian-inspired-tripod-walking-ro...