Are you specifically trying to run re3 with the Japanese SourceNext version, or looking for the SourceNext version of the game itself?
It looks like you're looking for a way to download (the reverse-engineered GTA III source code) specifically from SourceNext (a Japanese digital distributor) or perhaps the SourceNext version of GTA III to use with re3. re3 sourcenext download
Are you specifically trying to run re3 with the Japanese SourceNext version, or looking for the SourceNext version of the game itself?
It looks like you're looking for a way to download (the reverse-engineered GTA III source code) specifically from SourceNext (a Japanese digital distributor) or perhaps the SourceNext version of GTA III to use with re3.