how do I link my battlefield 3 account

That depends on which platform you are playing BF3?  If on PC it will automatically create a soldier the first time that you play.  If on console:

Here is how to do it on PS3:

Step 1 - Go to www.ea.com

Step 2 - Click on 'Login' in the upper right corner, if you are already logged in, log out.

Step 3 - Now the login form pops up, ignore the textfields and click the PSN button below

Step 4 - A new window pops up that allows you to login with your PSN account.

Step 5 - Now that the EA Server knows that you are the owner of this PSN account and if you already have an EA account you can now login to connect both accounts. In case you don't have an EA/Origin account yet, simply click the link on the right side to create one.

Step 6 - start BF3, at the BF3 main menu right click the d-pad to open your battlefeed and log in with the origin account that you used when you installed BF3.

For Xbox 360 onnect XBL with Origin http://battlelog.org/bl/tutorial_how-to-connect-your-xbox-360-account-with-battlelog_41

OR just start BF3, at the BF3 main menu right click the d-pad to open your battlefeed and log in with the origin account that you used when you installed BF3. 

C