Tensile Stress:
Tensile stress is a type of mechanical stress that occurs when a material is pulled or stretched in opposite directions. It is characterized by a force that tends to elongate the material and is typically measured in units of force per unit area (such as N/m² or Pa). Tensile stress is responsible for the deformation and eventual failure of materials under tension.

Compressive Stress:
Compressive stress is the opposite of tensile stress. It occurs when a material is subjected to a force that tends to compress or shorten it. Like tensile stress, compressive stress is also measured in units of force per unit area. Compressive stress can cause materials to buckle, crush, or fail in compression.

Shear Stress:
Shear stress is a type of stress that occurs when layers of material slide or shear against each other in opposite directions. It is caused by forces that are parallel to the planes of the material and perpendicular to the direction of motion. Shear stress is commonly encountered in cutting, punching, and shearing processes. It is also responsible for the deformation and failure of materials in bending, torsion, and sliding.

Forming Processes:
Forming processes involve the shaping or deformation of materials into desired shapes. These processes can include rolling, forging, extrusion, bending, and many others. In these processes, metals are subjected to various types of stresses to achieve the desired shape or form.

Explanation:
In the context of forming processes, metals are subjected to all three types of stresses mentioned above: tensile stress, compressive stress, and shear stress. Let's break down the reasons for each type of stress in forming processes:

1. Tensile Stress: Tensile stress is commonly encountered in forming processes when materials are stretched or pulled to shape them. For example, in processes like wire drawing or stretch forming, the metal is subjected to tensile stress to elongate it and give it the desired shape.

2. Compressive Stress: Compressive stress is applied in forming processes where materials need to be compressed or shortened. For instance, in processes like forging or stamping, the metal is subjected to compressive stress to shape it by applying pressure and reducing its height or volume.

3. Shear Stress: Shear stress is prevalent in forming processes that involve cutting, bending, or shearing of materials. For example, in processes like shearing or bending, the metal is subjected to shear stress as layers of the material slide or shear against each other, resulting in the desired shape or form.

Therefore, considering the nature of forming processes and the types of stresses involved, metals are indeed subjected to all three stresses: tensile stress, compressive stress, and shear stress. Hence, the correct answer to the given question is option 'D' - 1, 2, and 3.