Step 1: Picture the three roll stack.
A three high mill has three rolls arranged one above another, and only the top and bottom rolls are connected to the drive motor. This layout lets the mill roll metal in both directions without ever reversing the motor, which saves a lot of time and power in a rolling schedule.
Step 2: Track how the workpiece moves through the passes.
On the first pass the strip goes between the bottom and middle rolls, and since the bottom roll is driven, it drags the strip along, and the strip in turn drags the middle roll around by friction alone. On the return pass the strip is raised and passed between the middle and top rolls, and now the driven top roll pushes the strip back, again dragging the middle roll by friction from the other side.
Step 3: Identify the idler.
In both passes, the middle roll never receives power directly from the motor, it simply gets carried along because the moving strip rubs against it. That makes the middle roll the true idler roll of the stand, and its rotation is caused purely by friction with the workpiece.
\[ \boxed{\text{Middle roll}} \]