All 3 films all seem to have been shot in the course of a year or two

The films were released between 75-79 but from judging from how little Toby (Ham Larsen)aged it seems they couldn't have been shot over more than 2 years tops. He seems to be 7-8 in the first one and maybe 9-10 by the third one. Its also possible the second film and the third were shot back to back.