question1: what's your insurance libaility limit?

Question2: how much are they suing? They have to show cause and proof of the amount.

Question3: did you take pictures and have record of the police report of this accident?

Question4: what’s the statue of limitation in your state?

You need a lawyer ASAP instead of wasting time on this board. Your insurance can only do so much within the insured bodily injured amount. If the other party can show proof of loss that’s in excess of your insurance limit, you are liable for that amount. That’s why they send you this notice. Get a lawyer and since it happened more than 3 years ago, check statue of limitation.