In recent years, many top managers have deployed three at the back systems and it is very widely used particularly by national team managers. A three at the back system can also be classified by a five at the back if considering the two wing backs as defenders.
However, they are typically counted as midfielders in this system and three at the back necessarily means deploying three centre backs. And, 3-4-2-1 is the most widely used system among all the three centre back formations.

There are a number of pros and cons of using a 3-4-2-1 system. While most people typically believe that playing three at the back is basically a more pragmatic and defensive system, it is not exactly the right thing to say.
While this system ensures defensive solidity because of the presence of three centre-backs, it is rather more attacking than defensive because this is usually a more direct tactic.
With the evolution of football over the years, we have seen more complete central defenders in the last couple of decades. Modern-day centre-backs are technically much more astute than they used to be in earlier days and are much faster as well.
They are good passers of the ball and are not just restricted to defending. This has inspired managers to come up with three centre back systems with the defenders building from the back and defending at the same time.
The biggest advantage of 3-4-2-1 formation in Football Manager 2024, as discussed, is the presence of two centre-backs and two wing-backs which makes teams solid defensively.
A 3-4-2-1 system becomes really conservative when you lose possession and can become highly aggressive on the break. This formation is best suited for counter-attacking teams who like to hit their opponents on the break.
A huge drawback of this formation is this system can result in less possession of the ball unless all your players are top-quality. With a lesser team, this formation might not work if you want to dominate the ball.
However, this formation can work like a charm even with lesser teams if you are happy with less possession and want to hit your opponents on the break. The beauty of 3-4-2-1 formation is it is very much versatile and dynamic and can work with both top players or relatively weaker players.