Hi Poco

the c dial is actually for cyan / red. it rarley gets used in RA4 printing, What I meant was if you use all three filters at once all that will happen is the third filter value will be the amount of nuetral density you have added to the printing pack which will darken if you add and lighten if you take away

30yellow, 50 magenta and 10 cyan

this pack will give you 10 units of nuetral density or approx 1/3 stop
take away the 10 cyan and you will lighten the print by 1/3 stop
( I cannot think of any practical situation where you would want to do this)
in most ra4 printing you will only use the yellow and magenta filter

When I print cibachrome I have found quite often I need to use the cyan filter

0 yellow , 20 magenta and 10 cyan- cibachrome balance(new universal paper)

When I print cross process negatives I also have found I have needed to use the cyan filter

120 yellow, 0 magenta and 30 cyan - cross process RA4 balance

Just so you are aware you should always be only using two of the filters at any given time for colour printing.
I hope this helps