Final answer:
Three scientists - Robert Hooke, Theodor Schwann, and Matthias Schleiden - discovered the cell theory.
Step-by-step explanation:
The cell theory, which is one of the fundamental theories of biology, was proposed by three scientists: Robert Hooke, Theodor Schwann, and Matthias Schleiden. These scientists made significant contributions to our understanding of cells. Robert Hooke, an English scientist, first coined the term 'cell' and observed cells under a microscope. The German scientists Theodor Schwann and Matthias Schleiden suggested that cells were the basic units of structure and function in all living organisms. Later, Rudolf Virchow, also a German scientist, observed that cells divide to produce more cells, further supporting the cell theory.