What do you want to do by sanding? Refinish or restain?

50 grit is extremely rough for sanding wood to do either refinish or restain, unless the finish is heavy and you intend to try and restain.

To refinish, sanding with 150 grit is sufficient.

However, the three you have done will look different, so you will need to do all the treads now.

A random orbit sander is much quicker, in my experience that what may be a finish sander by your description.

Use the 50 on all, then use 80 grit, then 120 grit and finally, 150 grit.