The EA rep did not comprehend OP's question, hence the answer provided is incorrect.
If you have *already* successfully linked your XB360 or PS3 account to your Origins account, and it *already* shows up in Battlelog, then the steps to change the default Battlelog platform via Google Chrome (and possibly the other browsers) are as follows:
1.) Log into Battlelog
2.) In the upper left corner, to the right of the text "Battlefield 3" is your soldier avatar, mouse-over the avatar until the drop-down appears. This drop-down will have bold white text on a dark background with links for: "STATS", "WEAPONS", etc. in the left column, "DOG TAGS", "LOADOUT", etc. in the middle column; your user name, default platform, rank, and a full avatar picture in the right column.
3.) At the bottom of the right column in small, un-bolded text is a link for "Change Soldier"; click on this and then select the soldier you want Battlelog to use by default. All soldiers are listed by platform, so pick the one tied to the PC.
This should help with *some* of the co-op issues that many other cross-platform PC players have been having.