FM3 and your car.
With an FM3, you may leave the country without your car and there is no limit on the number of trips, etc. However, you should be aware that renewal of the FM3 is an annual event and that you must be in Mexico to do it at the same time each year. Here in Chapala, where immigration agents visit weekly, it takes about a month. It can be done faster in Guadalajara (in Spanish).