I actually liked Ong Bak 3. Compared to the first two there’s a whole lot less action, and a lot more dance and philosophy. It is a lot slower as a result, though it did a good job of tackling some of the Buddhist teachings about life in the process. It can be a bit awkward when warrior/movie types try to instill life lessons in their craft, though it seems to me that Tony Jaa, the star of the show, thought about his approach well.
The original Ong Bak gave a modern twist on Muay Thai, involving a lot of mind-bending acrobatics and huge amount of flying elbows and knees. Ong Bak 2 went back in time, and to an awful lot of weapons: knives, daggers, poles, spears, you name it. Ong Bak 3 tied the story of Ong Bak 2 to the original, and was much more about meditation, life, and what seems to me a blend of Tai Chi and Muay Thai.
All in all I would recommend the movie, as long as you aren’t seeking blood lust. It certainly made me think fondly on my time with martial arts, and dance. Jaa puts that the two are related, and both are a way of respecting and training the body. I can dig that, and I look forward to seeing what he comes up with next.