I am, honestly, slightly confused by the question wording, so here is what I think you need! :D
We can start by listing multiplies for each:
3:
3, 6, 9, 12, 15, 18, etc
4:
4, 8, 12, 16, 20, 24, etct
We can see that the least common denominator will be 12.
Answer:
If you need to multiply them:
But you do not need to use a least common denominator for multiplying and can go straight to this:
Hope this helps, have a nice day!