Never really read the paper books, but listened to the excellent audiobook version only a couple of years ago or so - I don't normally praise actors, but David Tennant really knows his craft. The third movie was never designed to be true to the story of the previous movies/series or even popular or "good", in my opinion.
I too wonder if there are young fans still - I'm so badly behind on HTTYD news, it's not funny. Will check out my previous haunts, and get back to this thread with my discoveries. If I'm not back within a week, please send a (friendly/trained) Deadly Nadder to find me.