All V2-Era routes and rolling-stock\loco packs work just fine in Run8 V3, but you'll need to pay attention during installation.

When prompted for the installation location, and assuming you have your V3 in its default location, all you need to do is change the "V2" to "V3" in the path. If your V3 is in a non-default location, you must specify the location. If you're unsure, you can right-click V3's icon, and Windows will show you the proper path to use.

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

After running your DLC installer(s), you will need to run V3's main installer real quick so that it can update and move the files as necessary.

Note: V2-era DLC installers all have filenames that are prefixed with "r8v2", "run8v2" or some such "v2" prefix.