R or S configuration of propane-1,2-diol
Your comment is correct where you assign the priority.

Rather than "rotating" or "flipping" the molecule, consider "looking" at it from the angle which makes the H atom be at the back.  In the graphic it is drawn to the left side, so place yourself on the right side and look down the bond.



The priority groups 1-2-3 run anticlockwise, hence it has the S configuration.