I was also trying to install ROS Melodic on the Raspberrry pi 3.
Added the stretch main distribution to the sources list, however it seems that it’s unable to find the package.
I’ve also checked the repository, and seems that under the
http://packages.ros.org/ros/ubuntu/dists/stretch/main/binary-armhf/
link there is an armhf repository.
So is the Pi3 supported in the end ?