I tripple boot windows 10, macOS Mojave and Ubuntu Mate 18.04 on my 2010 mac pro, but obviously because it's a mac, it's a little different. I use the refind boot manager to select my OS. It works on newer UEFI based PCs as well so you may be able to use it. Basically the computer starts up to refind, then I can hit g to boot the first detected instance of grub (or l to boot linux directly if that's how it's set up, some experimentation will be required here), w to boot into the first copy of windows and m for macOS. You can also hit one of the number keys to boot that choice from the menu, so you could figure out by trying it which systems occupy which spaces and use that method. It's a fairly clunky solution that won't even work if you have an older BIOS based computer, but it's worth a shot if not.

Grub will take precedence over windows' bootloader. You just go and edit the config file, set the default, make the timeout something reasonable so you can change if you wish, then save and regen the configuration.

If you install GRUB as your bootloader, I believe you can set the default OS within Ubuntu. The problem is once you've booted into windows, I don't think it will be aware of GRUB.