How to use iMac 2017 as monitor for MacPro M3

The 27” 2017 iMac never supported Target Display Mode, and the M3 MacBook Pro doesn’t support using any iMac as a Target Display.

The 27” 2017 iMac is also too old to be an AirPlay Receiver for AirPlay to Mac.

Review the document: Use your iMac as a display with target display mode - Apple Support

Target display mode is a feature available with late 2009-Mid 2014 iMacs. The display Mac has to be running macOS High Sierra or older and the signal producing Mac has to be running macOS Catalina (ca. 2019) or earlier.

This could only be done with a very few and very specific older iMacs and support was very limited. Read this topic for a 2021 perspective on this: Using iMac as an external screen - Apple Community

and this 2021 post by Allan Jones about software limitations:

TDM configuration - Apple Community

and read about Screen sharing Share the screen of another Mac - Apple Support