Hello, and welcome.
I'm sorry to hear that your ID expired 3 months ago. If the expired ID is a driver's license, can you try to renew online or make a quick stop at your local DMV to renew your license or at least get a receipt saying that you are renewing and waiting for it to arrive in the mail? This would help at Security.
The ID you show at TSA security screening must be up to date; however, this happens to many travelers and there is a list of ID's that are acceptable to the TSA in addition to a driver's license or passport (if this is what has expired). If you have any of these items and they are not expired, you can use them:
If you have anything else with your full name, address and a photo, that will help. You will need to have additional screening in which the security screener will search available databases to confirm your information and identity and this takes longer than the usual screening, when you have an unexpired ID, so arrive at the airport about 2 hours earlier, just to be safe and not miss your flight.
You should be able to fly if other sources of ID and your identity in the databases prove that you are who you say you are, but this will be up to the TSA security screener.
I wish you much good luck!
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