Multithreaded/async request and wait till they are all done and then process the results

У меня есть (хочу) выполнить поисковый запрос к нескольким источникам. Now i've done some multithreading in the past, but it was all fire and forget.

Now what i want to do, is to spin up 3 identical requests on 3 different objects, wait until they are all 'done' (and that gives me the first question: how do they say 'i'm done', and then collect all the data thet've sent me.

So in pseudo code i have this interface:

interface ISearch
    SearchResult SearchForContent(SearchCriteria criteria)

So in code i create the three search services:

ISearch s1 = new SearchLocal();
ISearch s2 = new SearchThere();
ISearch s3 = new SearchHere();

And then call SearchForContent(SearchCriteria criteria) on all three of them, in a multihreaded / async way

and the they all come back to me with their SearchResult and after they are ALL done, i process their SearchResult objects.

I hope these lines of text kindof makes you get what is in my head :)

i'm working on a ASP.Net 3.5 C# project.