running two operating systems

I believe the only option for the OP is to use a VM.

Any iMac that is running macOS 12.3 Monterey is far too new to run OS X 10.6.8 Snow Leopard.

Potentially, yes. Is your iMac an Intel or Silicon CPU equipped?

Regardless, you have a number of choices to consider.

They basically are:

1. Create a bootable USB with OS X 6.8. Then boot up your Mac with this USB.
2. Create a second partition on your Mac's internal drive. Then install this OS to it.
3. Use a Virtual Machine manager, like Parallels. You would then create a VM of OS X 6.8. Then run this VM when you need to use InDesign 3.

FWIW, I use the third option when I just need to run a particular app on occasion.

Thanks, you may be absolutely right in this situation. Currently, I don't have a Mac, running Monterey, to fully test this out ... and why I prefer the VM method.