How to make a 3 way call
How to make a way call
There is term for this, it's called conference calling.
When you phone someone on an android you should be able to phone another person after the first call by pressing the + icon on the call menu screen, then there should be a merge icon that should look like 2 lines merging to one with an arrow, that will merge the calls.
Edit: I don't know how these kinds of calls are charged, but I'd say you might be charged for two separate calls at once.
Hope this helps.