Comment by solid_fuel

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...