Hello and welcome. I'm not going to be any help with NetBSD, but perhaps I can help with the common configuration points among the 3 platforms, so that you only need one OS image that boots across them.
Assumption: you'll boot and use a USB-drive, so you can easily carry one USB device from machine to machine.- If you must use the 32-bit only platform, then you must use a 32-bit OS.
64-bit capable machines can boot in 32-bit. But not the other way 'round. You may have trouble with some applications. 32-bit x86 CPUs have fewer, smaller registers, limitations on memory, and other restrictions which might limit your OS's ability to deploy large and complex applications like Chrome, Firefox, or LibreOffice. Usually, the problem is compiling them, not running them if they have been successfully compiled.
- Use an MBR on the booting drive, adding an EFI boot partition.
The EFI partition (FAT-formatted, containing EFI boot-blocks stored as files) is necessary for booting via EFI.
- Also install MBR/PBR boot blocks.
In this way, the drive will be bootable from both EFI or MBR platforms.
Edited to add: The OpenBSD install media disk drive image is created this way, so that the media is bootable on the largest spectrum of systems.