A month ago I wrote a simple plugin, Constellation, for managing ST3 projects and workspaces. It has rough edges, but I've been using it daily for the last month and it is now available via Package Control.
It groups existing projects into a named "constellation", and can open/close all of those projects together. This is great for managing sprawling projects, but I've also found it nice to group by context (I open all of the active projects for a given client when I start a work day, and close them out at the end).
I always felt like I was swimming against the current with ST3's built-in project/workspace system, so I fell back on leaving everything open. This "worked", but the mental overhead and task-switching friction that came with having a dozen windows and hundreds of tabs open became an albatross. I'm really pleased with how much friction this has cut out of my workflow.