2 layer construction is the most common waterproof/breathable layer construction method. In this process, a laminate or membrane is applied to the face fabric, with nothing attached to the interior. A semi-loose hanging layer is typically added to provide protection (usually mesh or other such porous fabric).
A 3 layer construction features a laminated external DWR fabric as the first layer. The fabric is then bonded to a waterproof/breathable membrane in the middle, the second layer. A polyurethane (PU) film or lining covers the second layer, which acts as a shield. The directive of this third layer is to keep sweat and oils from congesting pores in the waterproof-breathable layer. As this third layer keeps dirt, oil, and grime at bay, the pores of the waterproof membrane are protected from clogging, thus maintaining better breathability between washings. The 3 layer construction features the highest performance.
For more information on 2 layer and 3 layer construction, please refer to our blog post: https://triplefatgoose.com/blogs/down-time/the-best-rain-jackets