Hi, I'd like to help you with your question.
If your child has not mentioned the incident since it occurred, then most likely she has forgotten it. At the age of 3, most children are unable to recall anything beyond a brief period of time. They have not developed to the stage of memory that allows them to hear or see something one time and be able to retain it.
If your child was older, say 6 or 7, this incident may be an issue. At that age, children can retain much more information, especially if it has strong emotions attached to it. Additionally, children at age 3 do not understand sexuality so they will not see anything odd about the behavior. A 6 or 7 year old begins to understand the differences between boys and girls so they would have an issue with it.
Although it is very unlikely your daughter will bring the incident up, if she does, let her know that the other child's behavior was wrong and that their mother or father will let them know not to do it again. Allow her to talk about it if she wants. Most likely though, she will forget it ever happened.
Let me know if I can help in any other way,