Triple Step

A child is running up a staircase with n steps and can hop either 1 step, 2 steps, or 3 steps at a time.

Implement a method to count how many possible ways the child can run up the stairs.

Examples:

n=1 → 1   (just 1-step)
n=2 → 2   (1+1 or 2)
n=3 → 4   (1+1+1, 1+2, 2+1, 3)
n=4 → 7