building with N steps, we can take 1,2,3 steps calculate number of ways to reach at top of building

Given n stairs, how many number of ways can you climb if u use either 1, 2 or 3 at a time?

When you find any difficult problem first step is to reduce it to very small problem with example

Consider 3 steps. you can take only one at a time. Then number of ways are 1: ie {(1,1,1)}

Consider 2 steps. you can take 1 or 2 at a time. Then number of ways are 2: ie {(1,1), (2)}