Sources state that various troubles in the production was the reason why the first plan didn't push through, but this time around, concrete plans for the production are being developed and the adaptation is likely to happen. In addition, since Part III lacks female characters, they're making sure to cast attracting male leads who will appeal, and they are keeping an eye on the fan reaction.
Take note, the rumor came from Cyzo Women's Blog, the same blog that states that a Fullmetal Alchemist film is in the works, which was later confirmed. In short, there's a possibility that this movie might happen.
[Source: Crunchyroll News]