Bah, it appears that rosdep doesn’t depend upon pip but expects it to be there 
. Try making python3-pip a build-package. That should unblock you short-term, but it’s something we can fix in snapcraft (or upstream rosdep) long-term.
But that’s not going to fix this:
You’re correct, rosdep doesn’t seem to have a rule for a Python 3 version of open3d. I suggest adding one.
Before you continue further down this path though, I want to reiterate a specific point:
Snapcraft changed how it did things for Noetic, and it currently is missing the magic that will take care of rosdep pip dependencies for you. I really recommend sticking with Melodic and embracing Python 2 for now if you can stand it, but that’s definitely a feature we need to add. Overall you can see that Python 3 is a pretty new look for ROS 1, and it still needs some creases ironed out.