Army of Two? Hah! Try Army of Two Plus One. According to Joystiq, you need only pre-order a single copy of SOCOM: Confrontation from Gamestop to pocket three beta keys.
Of course, if you plan to download the game from PSN, a quick (read: year-long) Qore subscription will also yield a beta key – but only one. And frankly, a $5 pre-order is cheaper, and fully retractable.
So, which route will you be taking?