jdotrjs has released a powerful 9-Slice Plugin for Phaser 3.
"9-slice scaling is a technique to deform a 2D texture allowing it to be scaled without deforming the corners. Usually this is done to retain the shape or embelishments on the corners."
It's a global plugin, meaning you can easily load it at run-time, or package it into your build process. The code to use it is nice and simple and there are examples provided and extensive docs.
Well worth a look if you wish to use 9-slicing in your game UI!