State laws and insurance company guidelines differ but it is not unusual for the insurance company to request that 3 estimates be done on the vehicle so that they can determine what the repair costs will be.
If you were going to pay for repairs on the vehicle yourself out of pocket you would likely want to make sure that the repair costs were correct. By receiving 3 estimates you and the insurance company should hopefully see 3 quotes that are relatively close together.
You would not want to get one estimate way high or way low or one estimate to list only a few things wrong while another lists several parts that need to be repairs. That is why 3 estimates typically will give an accurate view of what repairs need to be made and what the cost will be.
If you don't have time or don't have the ability to get 3 estimates, with some carriers you can request that they send an adjustor to review your damages and determine an amount, but even then you would want to verfiy that the amount they are paying you would cover the cost at a repair shop.
To find out what your state insurance regulator says regarding insurance companies and estimates, check with your state's Department of Insurance. For example the DOI in Utah notes that an insurance company may ask for several estimates. There is no UT law that states how many estimates you must submit or that limits the number the company may ask for.