Aligning the slice plane of the Slice modifier can be tedious, especially when you want it aligned to an arbitrary plane or the normal of a face rather than the world X, Y, and Z axes. Align Slice Mod Gizmo(Plane) to 3 pts is a 3ds Max script from miauu that handles this by aligning the slice plane to the normal of any plane defined by three points.
Using it is straightforward: run the script and pick three points. Snap is turned on automatically when the script runs. Holding the Shift key changes the behavior, aligning the slice plane to the currently active grid, or the home grid if no custom active grid is in use.
The script is at version 1.3.0, last updated in April 2023. This release fixed an error that appeared when the script was run with no object selected, and an error that occurred when a Slice modifier was applied to a Sphere primitive. It is listed among the entries on the Autodesk App Store.