Hey, thank you for your answer 
No, I’m using a value for each point straight way. My plan is to calculate a rough plane using those 3 points, use this plane to calculate all points that belong to it, and finally use linear regression in the whole list of points to fit the best plane.
But, again, I suspect that I need to account for lens distortion effect.
I found this thread in which this seems to be accounted for, but I’m not sure of how to apply it to my case.