July 10th, 2016, 14:02
You aim the installer at the root of P3D.
When you install into FSX you'll see that it aims at (typically) "F:\Program Files (x86)\Microsoft Games\FSX".
So for P3D you want to aim at something like this: "F:\Program Files (x86)\Lockheed Martin\Prepar3D v3".
This might not be exactly what you have on your system depending on the OS version but the general idea is there.
Aim at the root of the game, not some aircraft folder. If you can change the drive letter it thinks it wants to install to
you should be able to change everything in that destination path.