One day before those three months are up, her gut will send her a message, "Leave him. You are worth more than that." Sure she might get lonely but when she's alone and thinks back, she will realize it was just sex. Her accepting your "in three months" excuse will sound an alarm sooner rather than later. Trust me. She knows what's up. She just hasn't acknowledge and accepted it yet.
Two questions: was she your friend before you began fucking? Did you befriend her just to fuck her?