When printing RA4 or Reversal with a colour dichroic head we only use two filters, using the third filter will create unwanted ND which may be required but as PE stated use the ND filter on some enlargers for that purpose.
All the expensive Durst units I used had a ND filter for reducing light output.

There are many uses for the cyan filter , specifically in Cibachrome and in the past nuetralizing interneg film packs.
I have seen in RA4 applications for the cyan filter when selective burning is required or in JD Callows case when one cross process and prints RA4.