Pitch Perfect 3 (2017) is an aca-utely disappointing bum note to end on.

In The Craggus household, many films are held sacrosanct but maybe none, save perhaps now “The Greatest Showman”, unite the clan in happy song as much as “Pitch Perfect 2”; fresh, funny and toe-tappingly, finger-snappingly, irresistibly sing-alongable, the soundtrack rivals even that of “Guardians Of The Galaxy” for in-car entertainment supremacy. It’s into this crucible of expectation that “Pitch Perfect 3” pitched its less than perfect threequelness.

Three years after graduation, tired (but not really) of adulting, the 2014 Bardon Bellas find themselves reunited by a performance of the current generation, led by Emily Junk (Hailee Steinfeld). At a loose end, the girls are easily persuaded by Aubrey (Anna Camp) to use her father’s army connections to get a slot on a European USO show which isn’t (but is really) a competition to support DJ Khaled on his forthcoming tour.

Despite the fact its co-written by Kay Cannon who scripted the previous two films, “Pitch Perfect 3” feels like it was written by someone who’s never seen the first two films but has had them described to them. In the absence of a unifying idea, Cannon and her co-writer Mike White (“The Emoji Movie”, “School Of Rock”) open up the big bag of female comedy clichés and kind of crap all over the ending of the second movie. Out are the love interests from the first two movies, so out go Jessie (Skylar Astin) and no Bumper (Adam DeVine) – not too sad about that one – and in come comedic retrograde steps for nearly all the Bellas, but particularly Becca (Anna Kendrick) and Fat Amy (Rebel Wilson).

The hard-won solidarity and sisterhood of the previous film is tossed aside in favour of mean-spirited backbiting and freewheeling Rebel Wilson adlibbing which is more miss than hit this time out. The clumsy and ill-defined USO Tour/ Singing competition introduces potential rivals for the Bellas but fails to develop it in any meaningful way, eventually side-lining it – and musical numbers – in favour of a ludicrous retconning action-adventure plot involving Fat Amy’s estranged criminal Dad (an embarrassed and embarrassing John Lithgow) which even shunts Aubrey’s estranged Dad storyline (a more thematic fit for a “Pitch Perfect” movie) into an oops-we-almost-forgot mid-credits stringer resolution.

Nominally the film’s antagonist, Ruby Rose brings her usual mix of undeniable sex appeal and utter lack of likeability to a role which gives her little to do but sing a couple of songs and sneer, marking the extent of her character development. Even Elizabeth Banks and John Michael Higgins struggle to summon up their usual snappy snark when faced such poor material.

Director Trish Sie does a decent job but there’s no making a silk purse out of the sow’s ear of a script which takes the series’ strengths –  musical numbers and sisterly solidarity – and sets them to the side in favour of a silly, mean-spirited travelogue. “Pitch Perfect 3” as presented has clearly spent quite a bit of time in the editing room, with a lot of footage from the trailer not making it into the final cut and a few dangling references to scenes or conversations we don’t get to see. It’s out of tune with the other two films and it’s a real shame this trilogy ends off-key and flat.