Will Mounts be able to jump?

[Jennestia]

Jumping is a great way to get your character stuck in some place it shouldn't...

Only if the game is designed for it -- Most MMOs don't even allow you to bypass obstacles just because you can jump.

to be able to move in the third dimension?

The ability to change your direction from front/back to left and right is already 3-dimensional movement, you don't need to jump in order to have 3D movement, utilizing a joystick or mouse for movement alone is 3-dimensional movement.