Yep. So the basic idea is which steps can I be on as I take my last step:
a_n = a_{n-1} ($ If my last step is 1 $) + _{n-3} ($ If my last step is 3 $ )
Extra question: Write a recurrence for the amount of ways I can climb n stairs using 1,2,3... or n step(s).
Should be easy now...