
In MMA there have been plenty of fights stopped due to leg kicks. EA UFC 3 represents that and leg kick TKOs is a very real thing. Also, not only does it hurt you, but it hampers your mobility and ability to shoot for a takedown. It can also knock you down in one kick if you are in the middle of an action.

IRL there are plenty of different variants to the leg kick. But they all involve leg to leg contact. a "lower leg kick" would be considered a calf kick.
In UFC 3 you only have to worry about one type of leg kick: your leg health is right, or left leg, and that is it.
It depends on a few different variables. the power stat and perks of the kicker. the leg health stats of the opponent. is he moving away from the kick, or towards it? is he in the middle of an action?
A few solid leg kicks from a competent fighter would be enough to sway the fight in his favour. somewhere around 20 should end the fight (please correct me here, guys).
One important thing to mention is that it is blockable with the lower block funtion (the same buttons that block the body.)
- Block head
- Block Body/leg
So if you think he's going for a body punch and you block it, but he's actually going for a leg kick, then you would automatically "check" his kicks.
Checking his kicks inflicts significant damage on him. And there are numerous horrific incidents involving checked kicks IRL that reflects this (Silvs vs Weidman II comes to mind).
have fun playing!