Sounds weird that all three are having the same issue.
If it would be a hardware problem, it could be either Toner, Drum or Transfer Roller.
On Vertical lines it could also be the Fuser.
Though it sounds unlikely that all three gets the same defect,
Tried printing an internal page?
Our school district purchased a couple of Xerox Phaser 4600s toward the end of last school year and when everyone came back from summer they were having issues with the print quality of every single 4600 we put in place.
The problem was that there would be 3 horizontal dark gray lines on every single page printed from the printer. I cannot find an solution online nor have I been able to find a culprit on the machine itself. Has anyone had this problem and found a fix?