I’d be really surprised if there are holes already drilled or punched in the ribs of a -3 kit! More likely you will have the holes in the skin already, so backdrilling through the ribs won’t work becasue you’ll end up with random holes.
If there are no holes in either the skins or the ribs, I’d measure and pre-drill the skins so that the line’s of rivets are perfectly straight and perpendicular, then transfer that pattern to the ribs the way I suggested up above.
Paul