Rush to the Classroom

Sandeep lives closest to college. Yet he is late everyday. What an irony! And his classroom is on the topmost floor of the academic building of the college, to which he needs to climb $N$ stairs. So, in order to reach his classroom and not miss the first class, he jumps stairs while climbing. He can climb either $1$ or $2$ or $3$ steps at a time. But Sandeep is cleverer than you think. He has