Other method of triple booting is having multiple drives installed, such as you can have each build on its own drive and after BIOS splash screen press F12 and then select which drive to boot from.
I use to set up drives with dual-boot but it becomes a big mess when 1 of the builds gets trashed and needs to be reinstalled cleanly. So I myself install an OS to each drive in my desktop and then press F12 after BIOS splash screen and then select for which drive to boot from which I currently have Windows 7 and Windows 10 on my one system on 2 separate drives and default is boot to Windows 7, but if I need Windows 10, I can press F12 and select the other drive my non-SSD drive to boot from.
If you do go with the method of a triple boot on a single drive, I suggest cloning the drive when its finally configured properly for all operating systems or create an image of it so that you can quickly restore the drive back to clean working state if the drive is still healthy to take the image.
FreeDOS is an option if you want a OS that is almost exactly like DOS and supports most of the DOS commands and software etc.
http://www.freedos.org/ I have used this for when I need a specifically DOS OS, but dont want to have to connect my old USB/Floppy Drive or Connect an Internal Floppy Drive to install the OS to a drive. FreeDOS can also be installed to a bootable USB stick as well and you can boot off that USB stick and no have to mess with installing DOS internally to a drive.
*NOTE: Since you have a laptop likely limited to a single internal hard drive you could install XP and 7 to the hard drive and then FreeDOS to a USB stick and make for an easier build/setup. Additionally if you want something internal for a drive vs a USB stick hanging off the side of it if you have a MicroSD port you could also make a bootable MicroSD and boot FreeDOS off of that. I use my MicroSD port on my laptops as a convenient backup location for most important data in case the drive dies etc. But I have also used it for laptops without hard drives installed to run off of a 16GB MicroSD as if the MicroSD was a solid state drive, but slower than a SSD.
