Can you install Three OS on single PC?

Answer 1

In short, yes, a user may have three operating systems on a single computer.

A user may install as many operating systems as allowed by the partition table. A MS-DOS formatted drive (using Master Boot Record formatting) can support four partitions that can be marked bootable; in this case, the limit is four operating systems per installed hard drive. Computers can have multiple hard drives installed, in which case the limit still applies. A computer with four hard drives installed can boot from one of sixteen different operating system partitions, assuming that it has the capability to boot from a non-primary disk (most modern computers offer this option).

The previous paragraph assumes that the Microsoft Windows boot loader (NTLDR) is used. If GRUB (the Linux boot loader) is used, it can address any partition on any drive that it supports, potentially allowing a user to boot to dozens of operating systems on a single computer. The partition table will still play a limit on this role, as MBR will never support more than seven partitions in total.

A UEFI-compatible system is likely capable of booting far more operating systems on the same amount of hardware. In theory, this would allow 128 separate partitions, so the system could boot into a myriad choice of operating systems.

One should note that all operating systems must share the same physical space. This makes it unlikely that a 40GB drive would comfortably hold three or four operating systems (Windows 7, for example, needs up to 20GB by itself). Most modern drives are 500GB or larger, so three operating systems would have about 150GB of usable space if it were split evenly.