The 11v11 formation that is mimicked by the 3-1-2 is the 4-3-3.
The 9v9 formation that is mimicked by the 3-1-2 is the 3-3-2.
The 3-1-2 features three fullbacks (one center half with outside fullbacks to the left and right, typically slightly higher up the field than the center half), one central midfielder, and two forwards (one to the right and one to the left).
The 3-1-2 is an aggressive, attacking formation with the central midfielder as the focal point. In the attack, the outside fullbacks push into advanced positions, creating a five-pointed shape with the central midfielder in the middle. The central midfielder is the primary ball possessor and distributor in the 3-1-2. The forwards work interchangeably in advanced positions to create attacking space and to find gaps to exploit in the opposing defense. In the attacking third of the field, the midfielder and forwards form a fluid triangle shape with each player working creatively in response to the movement of the others. The center half serves as support for the attacking players, often dropping deep on the side of the field where the attack is operating to provide a release if the attack is well-defended. The center half often dictates the direction of the attack. In advanced positions, the outside fullbacks play wide on the wings to create open spaces for the midfielder and the forwards. The outside fullbacks also make attacking runs along the wings to support the attack, to increase the pressure on the opposing defense, and to cross balls into the box. When the 3-1-2 is employed deep in the attacking third of the field, its shape more closely resembles a 1-3-2.
When well played, the 3-1-2 creates a great deal of attacking space to exploit. The constant movement of the forwards and the aggressive actions of the outside fullbacks -- which leads to unpredictable and unexpected angles of attack -- creates pressure and confusion for the opposition.
The 3-1-2 is vulnerable to counterattacks. If possession is lost when the outside fullbacks are in advanced positions, the center half can be stranded as the only defense against a counterattack.
When to Employ the 3-1-2
The 3-1-2 is an effective formation for teams with highly-skilled, fit players who can handle the responsibilities of the central midfielder and center half and speedy players who can make the almost constant runs required of the forwards and outside fullbacks.