Yes, it is certainly possible to have 3 OS's or more on one machine. As with a dual boot system, you'd want to install Windows first, then your second OS(Ubuntu for example), then your third OS(Mint), and so on. This may require some reconfiguring of LILO or Grub, whichever you're using, after your final OS is installed.
As for disk space; fdisk from the command line or qtparted/gparted from the gui. Either of these should give you the info you need.