I have generally enjoyed the How to Train Your Dragon movies. They were clever and fun. I think a bit of the cleverness has worn off on the third installment. We are nine years removed from the original move, so I don't know if the audience is a rolling target. A six year old in 2010 would now be 15.
I think the best part of the movie are the sheep. They got one of the biggest laughs. The message echoes Toy Story 3, but is no where near as good at it.
I think the biggest flaws with the movie are Hiccup continue to work in isolation and bearing the weight of the world on his shoulders. He also never asks for help or takes more than thirty seconds to think through a plan. There is clearly a support network around him, but he relies on Toothless to get him out of any situation he finds himself in.
I assume young kids will enjoy this movie, but I don;t think it will resonate as well with the original movie's audience. Of the two animated movies released recently, this and Lego Movie 2, I think Lego movie is significantly better, despite having two large flaws. This movie just didn't do much for me.