@Vincent_Rabaud Is usually the one releasing OpenCV in ROS. I think that he would have the most context to tell us how much API breakage there is between these versions and if it makes more sense to release openCV3.3 in existing distros or to release it only in ROS Melodic.
Edit: opencv3 package version in all ROS distros can be found here: http://repositories.ros.org/status_page/compare_indigo_jade_kinetic_lunar.html?q=opencv3