I'm having trouble updating to macOS15. It downloads but doesn't complete the installation - gets stuck with 3 minutes to go. A

Installations of macOS may inform you that there are __ minutes remaining, and it can take far longer than that with little to no apparent progress. If you had sufficient storage remaining on your Mac, then it is a waiting game for the completion that is difficult for the impatient.

If you still haven't been successful in installing Monterey read and try the following:

I've had better luck from the Safe Mode because I evidently have some 3rd party software that interferes with the installation so try the following:

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support.

NOTE: Safe Mode boot can take up to 3-5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

#2 - go to the System/Software Update preference pane and begin the download.

#3 - run the installer from the Safe Mode.

Can second @VikingOSX has mentioned. Much like the Bug Sur before it - will take some time and several pauses will occur - normal. Countdown clock may say 15 minutes but in Real time may be 30 minutes. Same goes for clock at 10 minutes could be 20 minutes in Real time.

Patience and fingernail bitting.

There is no macOS 15. Are you trying to install Monterey?

or, are you trying to install iOS 15 on your iPhone?