my advice.. never try to lie or cover up something. Remember top MBA admission commitee hires some companies to investigate your background. And these companies do not just contact your recommender. Imagine that they give a call to your company, and somebody---no matter who, anyway not necessarily your recommender-- picks up the phone and tells the truth: that you left three months ago.... and if in your reference letter it is said that you still work there... you will see the consequence.
So just try to be honest. You can always explain why there is a gap, but you will never be given the opportunity to tell them why you lie.