According to vgchartz.com:
Pokepark Wii total sales: 1.13 million worldwide as of 28/12/13
Pokepark 2: 0.7 million
That's not all that shabby for a videogame in general, but rather poor for a Pokemon, and also factor in the Wii had a massive installed userbase, while the Wii U, er, doesn't..... also the sequel performing quite a bit poorer than the original..... so really, there hasn't been any news of a third game, and unfortunately I wouldn't expect any news either...