the m3 is part of the road of justice gang. it is the odd car in the gang. odd car meaning that when you first see the road of justice gang, you have to lose to all of them once; the m3 won't appear just yet. then after you do that, you begin to race them and after you beat them, the m3 will appear as a gang member so you can race against it and win it.
road of justice gang appears during the second half of txr2 after all zones are unlocked.