This third movie version of Kästner's delightful novel is a bit of an embarrassment. Filming it as a contemporary piece in 1974 (rather than going back to the 1930s and making it as a period film) was an understandable but problematic decision by the film makers. The 1955 version did not have such a problem, the required adaptations were then rather minor.
But society had moved on since then in Germany and in particular the life style of the rich and the poor and the tensions between them had undergone a complete upheaval; and some of these tensions are very much at heart of Kästner's novel. Thus to place the story in 1974 some changes to story lines and characters were needed. For example, the pennyless and unemployed academic Dr. Hagedorn (the romantic lead) would not ring true in 1974 and was consequently replaced with car mechanic Boris Dorfmeister; this was one of the less contentious changes, although I would not have chosen a blue collar profession for him. More problematic than Boris was the creation of the character of Titus, a completely failed attempt to replace the Kesselhut character (a butler) with something really modern; it was doomed to fail because the character Titus replaces is meant to be old-fashioned and living in the past, and Titus is the complete opposite - removing that aspect from the story leaves the relationship between the three men of the title unbalanced.
In the final result, all the elements that were designed to be hip in 1974 (like the collection of pinball machines) stick out like sore thumbs now, and make this film look much more dated than the 1955 version.