High hopes for Servo[1], but it'll still be years yet until it's able to challenge any of those three. Webkit2[2] will probably be ready sooner, but it's less of a radical departure than Servo's intended to be.
Oh yeah also, while I guess it's missing a lot, the code is actually very, very readable. That's nice.
https://github.com/mozilla/servo/tree/master/src/servo (the actual layout engine part)