GVWR = Gross Vehicle Weight Rating. In fact if you look at the plate on the door, it will list the front and the rear axle weights as well. 1947 lbs front and 1720 lbs rear in the case of the MP3.
In theory, you are not supposed to load the vehicle in such a way as to exceed that weight. If the curb weight of the car, with all fluids, is 2900 lbs then you can carry 3644-2900 = 744 lbs of useful weight.
2 people at 170 lbs each =340 lbs. 744 - 340 = 404 lbs of other cargo you can carry.
or
4 people in the car at 170 lbs each 680 lbs this means that we have only 60 lbs of cargo left.
You get the idea. I think Mazda gave the weight of the MP3 as 2725 lbs.