Ann, three months is quite short notice - we had a letter from a third party after an accident, a year after the event. Injury claims can be made up to three years after an accident.
Don;t get wound up by the letter, it's standard stuff. Tell your insurance company, send them the letter, that's all you need to do and what you pay your car insurance for. Presumably they have all the information to hand as you reported it when it happened? - if any solicitors are appointed it will be your insurance company who does it (and pays for it from the legal cover) - don't do anything off your own bat or you'll start complicating matters.
Solicitors' letters are perfectly normal (I used to work in motor claims). All you need to do is to respond to the letter - say no more than "I acknowledge receipt of your letter which I have passed to my motor insurers, XYZ insurance company". Say nothing else, don't pass comment about anything which happened, just send it on.
I hope this helps.