On startup the new version of Cura would start to open (evidenced by Activity Monitor process) then close. I decided to delete my existing Cura app data since (a) need to recalibrate the printer anyway, (b) getting an additional new printer anyway, and (c) didn't know what else to try.
Navigate to ~/Library/Application Support/ and look for anything Cura-related. In my case, there was a Cura folder with a directory for the two previous versions I'd had installed.
Go ahead and delete those, you didn't want them anyway.
My Cura 3.0 now starts just peachy.
I have a number of projects, both software- and hardware-related, coming down the pike right quick now. Think Raspberry Pi 0W, Raspberry Pi 3, NodeMCU, IoT, Elixir/Phoenix, Kafka, AWS, Nerf gun mods/hacks, and so on. I'm open for business in a new workspace, with new tools, and new mindset.
Let the games begin.