Install the latest ModManager from here:
http://fluffyquack.com/WIP/modmanager.rar

Change the game to RE3R Demo. If the modmanager says there is no RE3.exe file, you must create a fake one or make a copy of RE3Demo.exe and rename it to RE3.exe. Then, after installing mods, you will need to run the demo through Steam instead of Modmanager to get it to work.