Be more specific please. What kind of controler are you using? If you've played any of the other WC games the *roll* key should be the same. If you are using the mouse. Hold down the right mouse button and move it right or left in the direction you want to roll.
And if you're using the keyboard its 'Q' and 'W' to roll left and right respectively
With the J-stick it's button to combined with a left or right turn of the stick
You can find the control schemes for various versions of WC3 at https://www.wcnews.com/controls.shtml We also have manuals with more info in our game manual section.