I want to buy a second copy of Battlefield 3

You can have only one PC soldier per Origin account.  If you want to start a new PC soldier you will have to create a new origin account and then purchase a new copy of BF3 and if you want Premium or DLCs they too will have to be re-purchased again under the new Origin account. 

You can however make new console soldiers under your Origin account, you will have to make a new PSN or Xbox Live account (on Xbox you will have to also buy another Xbox Gold Subscription) and then when you install the game under the new gamer tag log in with your origin account and the new Level 0 soldier will be listed in your battlelog under your origin account.