hp% abilities are to help fight tanks and are usually found on champions that are meant to build tank, without the % damage they would do next to nothing later in the game (unless they had armor/mr scaling). Isn't true for all of them of course.
Although the 3 hit stack is more common now I think it is implemented in a unique way here that is unlike all of the others. Other champions when you hit 3 stacks it just did more damage or speed boost or whatever. On Tahm it changes how all your abilities work against that person. I really like it and think it will give him clear strategies and also counterplay because the enemies know to stay away if they get 2/3 stacks on them.