What's a triple boot method(macOS, Linux, Windows 10) that is safe and (relatively) easy to do?
You can install Windows via Bootcamp, Apple provides instructions for that. For Linux, follow these detailed instructions: https://www.lifewire.com/dual-boot-linux-and-mac-os-4125733.
You can basically install as many Linux distributions as you have space for.

I have a dual boot setup with macOS High Sierra and Windows (via Bootcamp) and a Linux distribution on a USB stick (permanent installation, not a Live USB stick), effectively making it a triple boot setup. I am using rEFInd as boot manager. The only downside I experience is that I have to hold down the option key every time I reboot my MacBook and choose the OS to boot from, because otherwise it finds no OS to boot and displays some EFI error screen. Don‘t know why and if it will be an issue for you but not a big deal either.