They are three extremely powerful champions. Janna is a very strong support with her shield and disengaging abilities. Mundo has one of the fastest clear times, a powerful duelist, and becomes a raid boss late game. Udyr is also a very fast and mobile jungler with potent ganks, and also a very strong top lane.
Adding onto that, Janna has a global passive that can make or break ganks since movespeed is surprisingly helpful for solo lanes. Mundo is very good at gluing himself to people trying to run, often helping to secure an extra kill when a teamfight dissolves. Since Udyr can be played top and jungle, there are a lot of people who will pick him early, see the enemy's top/jungle, have a teammate counterpick one of those roles, and play Udyr in the other role just as well.
