FWIW, Macintosh System 6 was just black and white (and only 640x480px), so 3D wasn't really tenable.
System 7 and 8 had support for color, and was as 3D as it could muster, like highlight and shadow treatment on buttons and other widgets, and embossed, draggable thumbs.
https://en.wikipedia.org/wiki/System_7#/media/File:Mac_OS_7....
Mac OS X brought Aqua, which was a pinnacle of 3D and skeuomorphic design. You know you're going hard for 3D when you add a drop shadow on all the text in your menubar:
https://en.wikipedia.org/wiki/Mac_OS_X_10.0#/media/File:MacO...
(and that IE logo in the dock wasn't there as a joke: it really was the best browser for OS X until Safari was a thing, years later)