IIRC raspbian breaks the armhf definition, the deb architecture is called armhf but the binaries are actually compiled for ARMv6 instead of v7 … given our core snap is a true armhf this will likely lead to issues where the systems overlap between snapd from the armbian deb and the core snap it needs to use to execute snaps…
Well, if you want to go with snaps all the way, obviously UbuntuCore. if it has to be deb based, i’d try if there is a proper debian armhf install (sadly there is no official ubuntu-server image for the pi3 yet)
core gets auto-installed alongside with your first snap … i guess you have some snaps installed on your PC …