In theory and practice any colour can be attained with the use of only 2 filters. When the third is used you generally only increase density and or the amount of red (if the cyan is the third and it isn't compensated) in the print. There is one exception (that I know of) when a third filter, cyan in this instance, may be employed or needed. That exception is when the range of the other two needs to be eclipsed.
The thing is though, one or both of the other two filters has to be set to zero so essentially you are only using 2 or 1 filter.
As an obvious example: yellow is at 0, magenta is at 0 and there is a need for 20 points of red. Other examples where cyan is needed whilst the other two are still being 'used' is when more magenta is required and yet it is already set to 0, adding equal amounts of yellow and cyan would create more magenta; or if yellow is at 0 and yet more yellow is required equal amounts of both m and c would be added.
Cross processed chromes, particularly fuji, and daylight film shot in harsh man made light (bars or nightclubs) are the two examples that come to mind.