Upvote:3
Your B1/B2 visa has an expiry date on it. That date is the last date on which you can enter the US. If you leave and return on or before that date then you may be readmitted. After that date the visa is invalid and cannot be used.
Immigration officials have the power to deny someone entry to the US even if they have a valid visa if they think they are intending to violate the terms of a visa. If you leave and quickly attempt to re-enter you are almost certain to be denied entry on the grounds that you are attempting to live in the US. It is up to the judgement of the official, but I would expect that if you have stayed for a year already then staying away for 3 months is not enough and you will be denied entry.
Note that if you are still legally in the US because an extension application is in process then you do not need to leave the US when your visa expires. Your visa expiry date is the last date on which you may enter the US. In fact I would expect that leaving the US would invalidate whatever extension application you have in process.