Has it all!
I think I love all the HTTYD movies and series more than my kids! Part 3/Hidden World couldn't have had a better ending for all of the characters. It summed up the genuine, respectful, loving, bonding, selfless friendship between the 2 main characters (Hiccup and his dragon, Toothless).
It also has Astrid, who is a positive female role model of a strait shooter, with brains, determination, strength, a go getter, who is loyal and supportive. I'd want her to be my friend if I lived in Berk. It is implied through most of the series to be Hiccup's girlfriend, but clearly a relationship that started based on respect, common interests, and FRIENDSHIP; not sexy stuff, and with a bit of kid level mindset of relationships preferred to be platonic but still affectionate - refreshing to see in this day and age.
The other characters are complex as well and add humor and plot through the series, but are less present in the overall story of Part3/Hidden World, as they should be, in order to really focus on the development and outcome of Hiccup and Toothless 's relationship...that helps Hiccup once again learn something valuable about himself.
All other characters add humor and plot and are complex as well.
There is tons of action, suspense, and adventure for boys, girls, and adult taste. Bad guys are not too scary. Arrows/fighting dont result in any visible harm. Language is appropriate. As a side note, the dragons have elements of other animal's behaviors, ex: cats, dogs, owls...which adds to the endearing natures of the personalities of the dragon characters.
Part3 has awesome colors/graphics.
I reccomend waiting to see movie Part3 after watching 1 and 2, since the story builds and continues through them all. If you like movie one I'd invest the time in Race to the Edge series before watching movies 2 and 3. Awesome series/movies for the whole family. I'm just sad it had to come to an end. Never tired of watching anything HTTYD!! Love, love love!!!