Triple boot- on a mid 2010 iMac11,3
Whatever happens, don't worry, because

You only lose what you haven't backed up™

So, if you don't have a backup of your system, make sure you create one:


Download and install "CloneZilla", a FOSS disk cloning utility, on a USB drive, in UEFI mode. Google is your friend, but this is a guide for you
Now attach a blank HDD the size of your hard drive and reboot your Mac in Clonezilla
Clone your internal hard drive over to the backup disk (make sure you don't do the other way around). The interface is pretty intuitive, but the procedure could take a while, regardless of how much data you have on your disk.


Now, make an ExFat partition on your disk using Disk Utility, launch the Windows installer, format it and install Windows.

Apparently in your case this seems not to be working and - in EFI mode - Windows messes up your EFI system.

I have a couple of suggestions.

The first: install Windows on a secondary HDD. This makes everything much easier, and you don't have to deal with countless other issues such as messed up partition tables and wrongly erased volumes. IIRC a secondary Hard drive can be installed in your system and - considering how cheap they are - it's really worth it.

The second, and most complicated (and maybe dangerous) is to run an Ubuntu (or any other distro) live session, open GParted, unmark your EFI partition as such (making it not appear as an ESP, but rather as a generic partition) and create two ExFat partitions. The first of them has to be very tiny (~200 MiB) marked as ESP, the other one - instead - should be as big as you want your Windows C: to be. After this is done, install Windows in EFI mode to the big ExFat partition (after having formatted it), then re-run GParted and re-mark the original ESP as such again. Now you can install rEFInd from macOS and boot your system in Windows by selecting the secondary ESP you created upon boot. If so you want, you can tweak rEFInd to boot an OS automatically by default, have a custom theme and do many other fancy things.

If you have no idea on how to do all of this, I'd recommend against doing it. Also remember that it's old hardware we are dealing with, so compatibility might not be the best, if existent at all.