How can I install both Windows 7 and Windows 8 on a Mac Mini 2012? I'm a developer, and I'm working on a cross-platform app that have to run on Mac OS X, Win7 and Win8. Unfortunately it's impossible to test my app in VM due to some hardware restrictions, so I have to install all of them on bare hardware.

I've bought Mac Mini 2012, but unfortunately I cannot find out how to install all 3 OSes on it. Boot Camp only allows to install single instance of Windows.

Is it even possible to have Mac OS X, Win7 and Win8 on the machine (with and without Boot Camp)?
I have done this myself. First install REFIt, an utility for dual/triple booting. It adds a useful selection of OSes on startup. It also includes a EFI/MBR fixing tool which is very useful. You might need to restart 2 times. Using the Disk Utility, add 2 more partitions to your disk. Format them FAT or FAT32. Restart your machine, and this time, assuming you have installed REFIt, instead of choosing OS X, navigate to the partition inspector (the hard drive icon). Just apply the settings it suggests. 

Reboot your mac and put the windows 8 disc in. The setup will allow you to format the partitions. Format both of the partitions. Install to the first partition. After installing drivers (optional) etc on win8, restart and install win7 to the other partition. 

Sometimes the Win8 will find Win7 installed and integrate the bootloader. That means that in REFIt, choosing either one of Win7/Win8 won't boot. Choosing Win7/Win8 from REFIt will give you the option to boot Win7 or Win8. Hope this helps.

NB: Your Windows partitions won't be named. REFIt won't know which is win7/win8. Or you can name it when you've installed the first Windows.