Hi BeepCount,
Thanks for checking in.
The EOS Rebel T3i uses an older way of interfacing with the computer and is not compatible with EOS Webcam Utility, regardless of which version of macOS or Windows you're running. I haven't seen anything to suggest that this will change with future versions of EOS Webcam Utility.
You can the list of compatible cameras by visiting the link below and scrolling to the compatible cameras section.
https://www.usa.canon.com/cameras/eos-webcam-utility
