Install Net Framework 3.5 SP1 Offline on Server 2012 R2

You can download the Windows Server 2012 trial version to get an ISO with the required source files.

Mount the ISO with the inbox ISO mounting and run DISM to install .Net 3.5:

dism.exe /online /enable-feature /featurename:NetFX3 /Source:d:\sources\sxs /LimitAccess

If it fails, also try to run this Update before trying to activate .Net 3.5:

Update for the .NET Framework 3.5 on Windows 8, Windows 8.1, Windows Server 2012, and Windows Server 2012 R2

http://support2.microsoft.com/kb/3005628/en-us

This removes 2 security updates which may prevent setup of .Net 3.5

This answer combines information from multiple answers and also adds some extra steps not previously mentioned.

I managed to do it in the following way:

1. Download the ISO image for Windows Server 2012 R2 (Note! has to be R2) from https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server-2012-r2 . It needs a windows live id registration and you need to agree to follow-up emails.
2. Once you've downloaded it to some location, transfer to the machine/VM you wish to install it to.
3. Copy it to another location within the target machine. This is to prevent "sorry, there was a problem mounting the file" error as described here, which relates to some transfer managers writing the file as sparse files. If you actually copied it already during the transfer in the previous step and not just mapped it, this step might not be needed: for me it was.
4. Right click the file in explorer and select "Mount". It should be mounted, I'm assuming to D: drive. If you get an error, revert to previous link.
5. Open cmd prompt as administrator and run the following commands (switches are explained in this other answer in a similar thread):

.

dism.exe /online /enable-feature /featurename:NetFX3ServerFeatures /all /Source:d:\sources\sxs /LimitAccess
dism.exe /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess

Both seem to be needed and in the order I've given. (source)

If you get an error "source files not found", reconfirm you have R2 image in use. That was the problem for me - the actual .NET 3.5 was missing from my image, which was plain 2012.

If both commands are successful, congratulations! You now have it installed. In case you're in doubt, you can confirm the version using steps outlined here.

Here's how i did it.

Download the ISO image for Windows Server 2012 R2 from https://www.microsoft.com/en-us/evalcenter/evaluate-windows-server-2012-r2 .

Put that Disc image on the computer that needs .Net Framwork 3.5

Right Click on the file and mount. This should create a D drive. Go to Server Manager, click Manage > Add roles and Features.

Click through to the Features selection and choose the .Net Framework 3.5 entry.
Keep clicking Next until you get to the final page.

Don't click Install yet- Click the small link at the bottom to "Specify Alternate Data Source." Change the path to be D:\Sources\SxS

Then install