Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I think the problem is likely to be either settings or software (and if software, likely the printer driver installed) on the Mac computer(s).
Please tell me how long the problem has been showing up with printing, and whether it has worked fine before this with these same computers. (Depending upon your answers on these two points, it might be one or another problem in one or another place.)
(*If* the problem has not showed up until today or very recently, I advise you to treat it this way:
1. unplug the printer, while it is On, suddenly.
2. leave it unplugged, and meanwhile, restart *all* the computers (that print to it.)
3. leave it unplugged from power, for at least 1 hour or so (roughly that much time.)
4. Turn it Off if the power switch is still 'On', then plug it in again to the power, turn it On if necessary.
Try it again and see how it goes. If it is producing blank sheets to Tray C when printed to from computers, and is only doing this recently or only today, then this may well help.)