An old rule of thumb advocated a triple superiority for the attacker in order to almost guarantee a successful attack. Historical examples for such 3vs1 offensives typically matched one of two basic models:
(1) Outflanking attacks where the attacker used superior numbers to create a longer line and win on one or two flanks.
(2) Frontal attacks with superior force density.
The greater the firepower (and especially its effective range), the less was (2) a convincing choice. The problem was especially evident at Kursk and during the Cold War. It was nevertheless necessary whenever the defender was able to put up a continuous, noteworthy front line (between two obstacles such as a sea, a neutral country or nearly impassable terrain).
Continuous front lines are not being expected for most future conventional war scenarios and weren't really relevant for (good) military thought back in the late Cold War either (two dozen divisions along a 1,000 km front were simply unable to man and defend a continuous front line).
Again, the question arose how to handle the defensive fight and avoid being outflanked by superior forces?
Thinking in the "lines" paradigm would suggest that we recreate the battle lines of the 18th century. An oblique order (counter-)attack or simply a full width delaying action would be possible in a 1vs3 situation.
We could alternatively look at the scenario from a non-linear perspective, assuming that the forces are more compact and able to turn quickly.
I recalled what I learned at martial arts about the fight against more than one opponent; reduce it to a 1vs1 fight by avoiding the second. The second attacker may move to your left, then you move to your right in order to keep him behind the first attacker who's in front of you. You circle around the first opponent by moving to the left if the second attacker threatens your right side.
It's surprisingly simple to keep the second attacker in a useless position behind the first one until you run into an obstacle or until the first attacker adapts his tactic.
This evasion of much of the enemy's attack power is interesting and effective, but its requirements are difficult to meet in battle.
The agility and reaction times of leadership and units put restrictions on this. Sadly, even resting forces cannot move very quickly into a new direction (although this can be much improved with proper training).
Even more reason for pessimism is being provided by the military historical experience about how difficult it is to disengage, to break contact. It's very difficult to pull back formations that are engaged in combat.
Forget the image of the nimble Karate or Taekwondo fighter for a second and think of classical Greco-Roman wrestlers. 1vs1, the entangle each other; it's almost impossible to disengage before the fight is over. A 1vs2 situation would ensure defeat, or a second attacker could easily move into a perfect position for his attack.
Sadly, the latter is a more accurate description of many modern land forces.
The great simplification in this text - modern brigades are more like a cluster of battle groups than a solid body of whatever shape - doesn't change the fundamental shortcoming.
Again, I'd like to emphasize the importance of battlefield or tactical agility - from brigade staffs down to Platoons. Reaction times need to be a fraction of those tolerated in peacetime training. Changes of orders must not create confusion and chaos, but rapid adaption and execution. Breaking contact on short notice with few losses is an important core skill for battle groups and brigades. It's not only useful if not crucial for the defence, but also of great utility on the tactical offence (more about this probably later).
One could expect that the well-funded Western armies train accordingly and are masterful at quick and agile manoeuvres at battle group to brigade level thanks to many, many free play exercises on diverse training areas.
The truth is more close to computer gaming. The enormously important training of (off-)road marches and actual outdoor tactical manoeuvres is being neglected. Such training happens mostly at unit, not at formation levels.
This is one of many reasons why our extremely expensive Western armies fail to reach their potential and to come close to the historical gold standards of experienced and slim wartime forces. Their excellence lies in the lack of threats who do it right at comparable funding levels.