This question already has an answer here:
This is a slight variation on an old puzzle that's likely already here, but be sure to check it before you mark it as duplicate.
What are the next three terms in this sequence, and why?
1 11 21 1211 1231 131221 132231 232221 ...
Edit: This is not the Look-and-say sequence as the duplicate links are. It is a variation on the idea of them, but it is not a duplicate in my opinion.