Too much is made of the formation business, IMO — oftentimes they're not rigid structures that must be strictly adhered to, but very basic outlines of the way you way to impose yourself on the game (or in a reactive sense, counter the opposition). There's no reason why a modern team with ample resources shouldn't be able to implement dos-tres-quatro formations in any given game — the key is to have a coach that can lucidly convey/implement his instructions to foster a sense of cohesiveness and automatism over time, and a group of players that are versatile and tactically astute in a balanced framework...so that they can seamlessly switch between apparent “shapes”.
e.g. Mönchengladbach can use a 4-3-3 or a 4-3-2-1 or a wide/wide diamond in-game on the basis of their 4-D-2 template under Rose and Marić — and they haven't even been at the club for long. And when you think about it, a 3-4-3 is not vastly different from a 4-3-3
if you have a defensive/holding midfielder who can double up as a sweeper, #8s that are competent in offense and defense, and fullbacks that can pretty much handle flank on their own.
https://spielverlagerung.com/2016/03/07/how-to-create-a-game-model/
The problem with United is not that we use different formations infrequently, but that we do not seem to be particularly well drilled or organized (even in primary shapes that are inconsistently executed), and the 3/5 man defense setups we tend to employ are frequently too restrictive or labored — especially when we use 3 centerbacks that can't carry the ball at a high level or pass between the lines to create wrinkles in the opposition's organisation, or wingbacks that are probably more suited to a side-centerback role in the setup. The coaching and selection of personnel needs to be be much better/eclectic.