Final answer:
The three major functions of a cell membrane are defining the cell's borders, selective permeability, and cell recognition through surface markers.
Step-by-step explanation:
The three major functions of a cell membrane, also known as the plasma membrane, are its ability to define the cell's borders and protect its contents, its selective permeability which allows some molecules to freely enter or leave the cell while restricting others, and its role in cell recognition through surface markers. The cell membrane acts as a barrier, separating the cell's internal environment from the external environment and regulating the transport of substances in and out of the cell. It is made up of a phospholipid bilayer with embedded proteins that facilitate various functions including transport, cell signaling, and cell adhesion.