Scroll down the above post. The post contains a picture of the lock out bar for the 3-point and where it goes.
The 3 point lock out bar "locks" the 3-point into the elevated position. When the 3-point is locked into position, the hydraulic system then powers the front center hydraulic cylinder - normally the mower deck.
Depending on which cylinder, center or the 3-point, you want to use, you can lock out the opposite cylinder. This makes the operation of which ever cylinder you are using much faster. If you don't lock out one cylinder, the hydraulic system will provide fluid to both cylinders - result is that the cylinder with the lightest load gets fully elevated before the cylinder with the lightest load.
If you are using the 3 point you do not need the lock out bar. But you need to lock out the center hydraulic cylinder.
Edit: Does you 154 have a diverter valve, located on the flat panel in front and below the seat. If your tractor has the diverter valve, you will probably not need the 3-point lock out bar.