I have a list of documents that are all tied to terms in the term store. I'd like to figure out a way to create a 3 drop down lists with the 3 term sets. The reason I want to do this is so I can pass the parameters from the drop down lists to the list of documents and only return the documents that match the parameters. What is this called? And do you know of a tutorial on how to do this?
If I understand your question well, you want to see documents that are related to a tag or its parent. TThen if I am right, you can do it using content search web part and term deriven Pages. Here is some example related to term deriven page and content search. By this way you can collect all the documents/items that related to a term or its children. But if there is only one list you can activate Navigation and Filtering Feature first at site level and then at list level and try to response your need by this simplest way. If I haven't understand the question well please clarify it for me. Thanks in advance!
You can create a filter web part and then connect it to an out of box List View Web Part. The details of how to do that are a bit much to put here, but here is a reference to a Microsoft post that describes it in exhaustive detail:
Note that as it says in the title, this is only available on classic pages, not modern pages.