Based upon the responses the assumption is the question pertains to B&W. I'll assume the opposite.
For colour printing you can achieve colour balance with any combination of 2 of the three filters: Y+M, M+C, Y+C. My understanding of why Y+M is the preferred choice is because cyan is a denser filter and will add more exposure, but it is just as usable as Y or M.
At times you may find a neg that requires more red than you have available with Y+M and therefore will need to employ all 3 filters.
Cyan can not be used as a ND filter in colour printing unless a corresponding amount of M and Y are added. Simply adding cyan will cause the print to be redder. I assume that when printing B&W cyan acts as nd.
Some enlargers come with a nd dichroic filter.
Hope that helps