how can i find a player that plays bf3 on xbox360 and i play on pc?

Keep in mind that not all console players have soldiers on Battlelog because in order for a console soldier to show up and be tracked via Battlelog the console player must link his PSN ID or Xbox Gamertag with his Origin account.  If they fail to do this they will not be searchable on Battlelog.  Once a console player links their console ID with origin you can use the soldier search function on the right hand side by clicking on the magnifying glass above your friends listing.  Type in the player's gamertag and then Battlelog will show him.