Question

Why the American Revolution was not as brutal as the French and Russian Revolutions.

Answer 1

All those revolutions are or were fighting an enemy with nowhere to go. Both sides have to either win or die. 

The American Revolution was against the British who lived far away and at some point found the cost of fighting across an ocean too high for the expected return and decided to stay in the comfort of their home.

There is no home comfort to return to when you fight at home, so you keep fighting knowing your enemy is on the same predicament so you can't expect an easy surrender.