At one improv show I went to, possibly the only improv show I ever went to, the audience was invited to shout out locations ("a bathroom," "a movie theater") and then we were asked to shout out some sort of emotion (love-struck, nostalgic, elated); then the improv actors went with these suggestions and made up some sort of scene.
At one point at that improv show, one of the actors had some vision about the direction this scene should take, but then another actor cut in more forcefully with a vision of his own, so the first improv actor had to quickly change course and go along with his colleague's new vision.
In addition to having limited experience with acting and improv, I also have limited experience with 3 year olds. With my lack of experience not being a barrier to opinion, I think that 3 year olds are masterful improv actors.
At my daughter's preschool, they have a play area filled with various costumes including a tutu, a chicken costume and an asian inspired coat of some sort. Somehow, my daughter and her classmates thought that these were the perfect set of costumes to go together and they figured out some scenario where they played together and it all worked.
At home over the weekend, my daughter was pretending to be a mommy cat while I was supposed to be her baby cat. When I announced that it was time to wash our hands, she said how much she appreciated me and that I was being such a good baby cat by reminding us all to wash our hands.