Verify the first and second mirrors are already aligned
Place the third mirror in its intended position
Set the third mirror to the correct height
Set the third mirror to the correct angle
Adjust the third mirror so the reflected beam or image hits the target point
Move the mirror slightly left or right until the reflection is centered
Move the mirror slightly up or down until the reflection is centered
Tighten the mirror mount without shifting its position
Recheck the alignment after tightening
Repeat fine adjustments until the reflection is stable and accurate