Sorry, I was assuming you had actually made a third Regional Dex. All you've done is added a line to an array.
In pokemon.txt, about half of the species have a RegionalNumbers= line. These numbers say what their Regional Dex numbers are for each Regional Dex. Each line has 2 numbers in, which means they are in (up to) 2 Regional Dexes. By default, there are no Pokémon that are in the third Regional Dex, and if that's the case, the National Dex is used instead.
You need to add Pokémon species to your new Regional Dex by editing pokemon.txt.
It'll work once you actually do what you want to do.