3 month is a little tight, but if you spend 2-3 hours a day you can get something.
Start with audio only courser, I suggest:
Michel Thomas - Foundation Mandarin (8 CDs)
Pimsleur - Mandarin I, II + III
Do both at the same time. Then listen to some Madarin on the radio, or on Youtube.
ChinesePod.com is also not too bad.
I guess you are not interested in Characters - never mind, do them anyway! Not to use them, that would take more time, but to have one more link that helps you to remember. Some people go as as far saying, learning Mandarin with the Characters is easier.
A very good book (if not THE very best) is "Learning Chinese Characters" http://www.amazon.com/gp/product/080483816X
It teaches you in a way that helps remembering by using both, R + L brain. It teaches the 800 Characters of HSK 1
The BIG problem with Mandarin is that it has only about 400 different sounds. So to a foreigner everything sounds similar, that makes it much harder to remember it. Otherwise the language is very simple and maybe the easiest I can think of.