must be pretty intresting to see her run. makes me wish we had a flash of her running

not completely sure if a third leg improves running by 50%. it's hard to imagine someone running on three legs, since there are no real examples

I think it would; in the same time as a normal person running pushes off against the ground twice, she does three times, so she's being propelled forward with 1 1/2 times the overall force. Since she doesn't weigh 1 1/2 times as much, she should end up moving faster overall.

it's kinda complicated, it's the same force applied as someone with 2 legs would at a time.
Rhythmic running would increase the impulse time by more than 50%, because by dividing the time between 3 legs also decreases the airborne phase, thus needing less time to deal with the impact of the steps. It's probably less tiresome than running with 2 legs, but the speed gain would be mostly from decrease of the time being de-accelerated by the air friction, which doesn't account a whole lot at sub-60kph speeds.
For running extra fast, an inner-leg/outer-legs sequence could be more effective, because you ensure half of the steps are twice as strong as a normal person's.

It seems to me that extra mass would result in an increase in inertia, and therefore an increase in speed after some time to accelerate.