What's a triple boot method(macOS, Linux, Windows 10) that is safe and (relatively) easy to do? I originally thought it was as simple as using Bootcamp and then making a partition for Linux. But apparently installing Linux breaks the Windows installation.

I currently have High Sierra installed on my rBP 13" 2014. I wish to add Linux Mint 18.3 and Windows 10. What would be the general procedure?

Below is the output from the command diskutil list.

/dev/disk0 (internal, physical):
#:                       TYPE NAME                    SIZE        IDENTIFIER
0:      GUID_partition_scheme                        *251.0 GB    disk0
1:                        EFI EFI                     209.7 MB    disk0s1
2:                 Apple_APFS Container disk1         250.8 GB    disk0s2

/dev/disk1 (synthesized):
#:                       TYPE NAME                    SIZE        IDENTIFIER
0:      APFS Container Scheme -                      +250.8 GB    disk1
                             Physical Store disk0s2
1:                APFS Volume macOS                   16.8 GB     disk1s1
2:                APFS Volume Preboot                 22.3 MB     disk1s2
3:                APFS Volume Recovery                517.8 MB    disk1s3
4:                APFS Volume VM                      1.1 GB      disk1s4
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.