To elaborate on Pabz0r's post, the game's beta does not start till Tuesday (for preorder folks). You will receive an email from Dice and Ea regarding getting access to the Beta. Don't believe me?
I PREORDERED BATTLEFIELD 3 (PC DIGITAL DOWNLOAD VERSION) ON ORIGIN. HOW DO I GET EARLY ACCESS?
You will receive an email from us with instructions on your early access. The cutoff date for receiving early access by pre-ordering on Origin is September 25th.
Just be patient and remember to check your email regularly.