can you play as any of the races in the campaign? or are you stuck with human?

couldn't get into the first game because of that, and was wondering if the 3rd allows all your Hero/Units to be any of the races.

I think you're initially allowed to play as the starting race of your hero, but I may be a little confused because I picked high elf for my hero and that's the first race you encounter. You can make allies with other races as you go that lets you use those races in-game.

Only the first game restricted you to humans.
In the following games you chose the hero race which then dictated your army race, other races becoming unlocked based on your actions:
In WBC2 you had to conquer their capitals. In WBC3 you also have the option of allying with them (some races will always be allied, others you can improve your standing with by doing specific missions). You then have the option to change race before each battle.