what are the three methods by which HIV and AIDS are usually transmitted?

Answer 1

Without seeking references, unprotected sexual intercourse and sharing needles are two methods that come to mind.

I searched Google under the key words "HIV transmission" to get these possible sources:

http://www.google.com/search?client=safari&rls=en&q=HIV+transmission&ie=UTF-8&oe=UTF-8

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.