They all incorporate great kicks, but TKD is HEAVY on the kicking.... alot of TKD schools will train a whole class on just 1 or 2 kicks. I find that kind of boring but in TKD thats the way its done sometimes...kick, kick, kick. As an instructor in TKD that has visited many schools I feel that TKD offers the best for kicking.... but not much for hand techniques, I'm pretty sure Kenpo would be the place to go for hands between the 3 you are looking at but TKD definately for kicks. We train speed kicks, power kicks, sneaky kicks..... every type of kick from every angle. I took karate for a while too and all I can say is... almost every karate school is different, some train like TKD and others do something completely different, like grappling. I'm thinking Kenpo is more standardised?
After saying all that, if your looking outside the 3 arts you mentioned, Muay Thai has the strongest round house type kick I've ever trained or been hit with. When kicking with the shin the difference is like a baseball bat compared to a fist. (shin instead of top of foot/instep) However Muay Thai doesnt have alot of different kicks in their arsenal, instead they have knees, elbows and boxing.
Damian Mavis
Honour TKD