I have already installed Linux Mint on Windows 7 so now I have dual boot with Linux Mint and Windows 7.They are both on diferent partitions.I have 1 free partition left.The question is how to install Ubuntu? Should I just run it from CD and install it on the free partition?Would I be able to run then all 3 systems (Ubuntu, Mint, Windows) from the menu when starting a computer or not?How to do that? 
How to install Ubuntu on a system where is already Mint and Windows 7?
ubuntu;linux mint;dual boot;window
No dual boot is a misnomer, it actually means multiple. You can have as many OS's on a system as you have room on your HDD(s).https://en.wikipedia.org/wiki/Multi_bootIn general when setting up dual boot systems you'll need to use one of the distros boot loaders such as Grub (linux) or NTLDR (Windows) to manage the various OS installations that you've laid down on the HDD(s).