I have Battlefield 3 and Sims3 on my origin account. My gf wants to play Sims on her pc, and I want to play BF3 online on my pc at the same time. Is it possible?
It might be possible, but I'd recommend you just spend the extra to buy another license.
Your Origin account (including the games on it) is for your personal use only. Anything more is a violation of their EULA. So no, it is not possible without violating the license agreement.