The three tenets of the cell theory are:

1. All living organisms are made of cells.
2. Cells are the basic unit of structure and function in living organisms.
3. Cells arise from pre-existing cells.

These principles were developed by several scientists, including Robert Hooke, Anton van Leeuwenhoek, Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. The cell theory is considered a fundamental theory in biology and has been expanded upon with modern scientific advancements.