There are not 3 main layers in mature residual soils, but rather 4. These layers form due to the weathering of bedrock over a long period of time.

Here are the 4 main layers:

1. Saprock: This is the layer closest to the bedrock. It is partially weathered bedrock, still relatively hard and solid.

2. Weathering Zone: This layer is above the saprock and consists of highly weathered rock fragments. The size of these fragments decreases as you move further up this layer.

3. Laterite: This layer is rich in iron and aluminum oxides due to the leaching of other elements. It is usually red or brown in color.

4. Soil Layer: This is the top layer of the residual soil. It is made up of fine-grained material and organic matter.

It's important to note that the presence and thickness of these layers can vary depending on the type of bedrock, climate, and other factors.