Do you know how to Triple boot Windows, Linux, and Illumos OS in the same PC?

I think it is already explained several times on several different sites. Just partition your disk(s) and install. Probably windows first
It may depend on your setup, so would be nice to explain some details about your hardware, OS versions and issues (if any).

Sure, nothing easier than that.


First, take devuan i386 live DVD to make four partitions. NO EFI
Take regular DOS / Legacy partition.
Example:
part1 100 GB
part2 100 GB
part3 100 GB
part4 100 GB

It is very easy you install windows 8 with DOS/Legacy MBR (no EFI) on your machine, example on partition 4.
You let the windows upgrade from win 8 => win 10. Yeah, you have MBR that works with Windows ! wow.

partition 1, you place netbsd with grub2, you boot and install it. netbsd will deal with your multiboot like magic.
Edit /grub/grub.cfg

partition 2, Linux, whatever is ok. take devuan, it is way cleaner than debian or ubuntu.

partition 3. Illumos OS

it takes a little bit of time.

It is super easy.