In your V3 folder, in the same location as V3's exe, create a dummy text file using NotePad called:

Run-8 Train Simulator V2.exe

Attention: Achtung: Vnimaniye: Make sure you don't leave the ".txt" in the filename! This can be prevented by changing the Save As Type to "All Files" instead of "Text Documents(*.txt)".

When you run the DLC installer, point it to V3's location instead of the V2 location.

V3's default location is C:\Run8Studios\Run8 Train Simulator V3