I thought that was an interesting question so I went to look at the API documentation for the Stream interface. And the method you asked about uses a Collector object, so finding out about Collector seemed like a good idea.
And when I clicked on the link I found that the documentation for Collector started out at the top with a description of those three type parameters. Did you find that?
(By the way, you posted this in one of the certification forums. Was that related to a particular certification exam? If not, let us know and a moderator will move it to a more suitable forum.)