Why did Windows 95 setup use three operating systems?

I never thought of Windows 3.1 as an OS. The other 2 was MS-DOS and Windows 95.

Agree, the terminology in those days was “shell”.

Though Windows 95 was arguably similar running atop “DOS 7” it actually imposes its own 32-bit environment with its own “protected mode” drivers once booted. Dropping to DOS reverted to “real mode”.

So did the lastest Win3.1 for workgroups, just MS spared all the fanfare for Win95. Not sure if the 3.1 version in the installers does.

Windows 3.1 was just a graphical shell. All the drivers and stuff were still managed by DOS. You still needed to configure your system with config.sys

EDIT it’s coming back to me. Windows 3.1 did have a a subsystem for running 32 bit apps called Win32 I think that’s what you mean. This was very much in the application space though.

It still used cooperative multitasking and Win 95 introduced preemptive.

I think it’d be fair to call it more than a shell. It was also a set of libraries that implemented the common user interface elements of Windows apps, similar to the Macintosh Toolbox but not in ROM.

Not sufficient on its own to qualify it as an OS though. The VM subsystem described in sibling comment makes a big difference though!