def stepPerms(n, memo=None):
    if memo is None:
        memo = [None] * (n + 1)
    if n == 0:
        return 1
    if n < 0:
        return 0
    if memo[n] is None:
        memo[n] = stepPerms(n-1, memo) + stepPerms(n-2, memo) + stepPerms(n-3, memo)
    return memo[n]