(x,y) origin for 3 wheeled robots

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I have a 2-differential drive wheels in front, and caster in rear, of my rectangular platform. I haven't been able to find a definitive answer on where (x,y) = (0,0) should be in the footprint parameter in the costmap yaml files. Is it the turn pivot location between the 2 drive wheels or the physical center of the robot frame. I have seen both used in different posts. I am using ROS Melodic.

Thanks