Very good! You mainly need to work on punctuation.
Hi! How are you? It's been a long time! I haven't seen you since 3 years ago!
I might add...
I haven't seen you since we went to that wedding 3 years ago!
"Since" is used to measure from a specific time or instance, so we would use it with one particular occasion or event in mind. Therefore it is usually used when naming a specific event.
Since your graduation
Since we went to that movie
Since we ran into each other in Chicago
Since my birthday
If you mean more generally that it has been about 3 years, we would use less specific wording like...
What has it been? 3 years?
It's been about 3 years
I haven't seen you in 3 years