+1 booting from SSDs.
In theory I'd just install the oldest version of Windows on one drive or partition, then the next oldest, then Linux. Linux ends up handling the bootloader.
In practice though, why do you want a triple boot situation? I also dual-boot and it's a pain having some resources in one OS and some in another (and I too have a shared drive), to divide your resources into three just seems like an exercise in masochism.
Can you not make do with virtual machines for at least one of these operating systems? The differences between Win7 and Win81 are so minimal that my only guess why you're wanting to do that is that Win81 is still supported with security updates.
Here's my OS situation:
SSD #1: Win10 gaming (and a couple of apps that require hardware access for recovering data - work related)
SSD #2: Linux
After that I have two VMs on Linux: Win7 and XP. The XP one isn't strictly speaking needed any more, once I made the decision that I had enough room on the Linux SSD then I moved the hard drive image for Win7 over to it and so it boots about as fast as the XP image does. Sometimes it's just handy having a Windows install easily available without having to reboot, I have a couple of apps in there that in theory could be handled in Linux with Wine, but since I found that MS Access 2000's font handling wasn't great with Wine, I kept my Wine usage as basic as possible.
Occasionally more VMs are spawned whenever I want to play with other Linux distros (Kubuntu and me are not a perfect fit, but perhaps I'm a fool for looking for a better fit, Win10 certainly isn't for me! Win7 and Lubuntu 18.04 LTS were).