center ch. as LCR ?

Sometimes speakers designed for movie center channel use have their frequency response tailored to enhance dialog intelligibility. Such a speaker would be a poor choice for all around use, and also a poor choice for center channel in a music system. Success or failure of your scheme will depend on the detailed design of the paradigm CC-300.

Your trio of centerchannel speakers will do just fine as L/Rs but will have the correct hi-frequency-dispersion pattern only if they're arranged horizontally. Ever notice that some 'center-channel' speakers have their tweeters (and MR drivers, too) mounted on a board that can be rotated 90 degrees? That's so the speaker can have correct dispersion whether mounted horizontally or vertically. If you use vertically a centerchannel speaker designed for horizontal use, the sweet spot will be very narrow.