What to do if my new vehicle3 months is considered total loss after an not my fault accident?

Assuming the car is a total loss, you can recover the then-current value of your car--NOT the purchase price, however; what it was worth at that moment, given depreciation, mileage, etc.--from a combination of your insurer, the at-fault driver, and the at-fault driver's insurer.
First, see what you get from your insurer (assuming you have the relevant insurance; i.e. collision). Then you can sue the at-fault driver (you sue the driver, not their insurer) for any amounts not paid by insurance. In addition to the value of the car, you can also recover costs directly caused by the accident, like car rental for a reasonable period of time while buying a replacement car, to the extent such are not paid by your own insurance.
Example: say that then-current value of your car is $20,000 and you had a $1,000 deductible; say you also incurred $300 of car rental after the accident. If your insurer pays you $19,000 ($20,000 less the deductible), you could sue the at-fault driver the other $1,000 and the $300 of car rent, or for $1,300. If you can prove the other driver was at fault (such as by testimony and/or the police report), you can recover that $1,300 from them (their insurer may pay for them).