There's no demand for a third team in NY. They need to do one of two things, it's pretty simple.
One, institute a salary cap of anywhere from $80-$95 million. This will never happen because the contracts already in play would have to be restructured and the players union would never, ever, ever let that happen.
Two, increase the penalty for going over the luxury tax. What the Yankees are paying now is a mere pittance to their overall revenues, and in no way is a deterrent to their spending. MLB should have the penalty be $5 for every $1 over the tax, or some factor like that. This has a much, much larger chance of passing because let's face it, only one team would be affected by it.