Why three documents
Because they answer three different questions, and a single document that tries to answer all three gets read by nobody.
The Style Guide Quick Reference answers how do I write this. It is short on purpose. A style guide that requires a table of contents will lose to whatever the writer already believes.
The Communications Handbook answers what are we trying to do. House voice, audience, the declared WCAG 2.1 AA accessibility standard, and the reasoning behind each — because a rule without a reason gets ignored the first time it is inconvenient.
The Communications Procedures answer who does what, and in what order. Role-based routing for every request type, so the question of who approves a thing has a documented answer instead of a hallway one.
What it changed
Before the set existed, house style lived in the heads of whoever had been there longest, which meant it was inconsistent, uncontestable, and unteachable. Every disagreement about a serial comma was really a disagreement about authority.
Writing it down does not end the argument. It relocates it — from the draft, where it costs a deadline, to the guide, where it costs one conversation and stays settled.
What happened: TODO — adoption across divisions, or the recurring argument that stopped happening once the guide existed.
More work on this page, and more about me over here.