Download and install from http://www.microsoft.com/en-us/download/details.aspx?id=34595. You need Windows 7 SP1 though.
It’s worth keeping in mind that PowerShell 3 on Windows 7 does not have all the cmdlets as PowerShell 3 on Windows 8. So you may still encounter cmdlets that are not present on your system.
The latest PowerShell version as of Sept 2015 is PowerShell 4.0. It’s bundled with Windows Management Framework 4.0.
Here’s the download page for PowerShelll 4.0 for all versions of Windows.
For Windows 7, there are 2 links on that page, 1 for x64 and 1 for x86.