By definition, the 3 parts to cell theory are:
"All living things are made of cells." This means that all living things must be composed of cells. The cells are the very foundation of the functions of every living being.
"The cell is the smallest living thing that can perform all the functions of life." Since only living things are composed of cells, it makes sense that the cells themselves are living things.
"All cells must come from pre-existing cells." All cells come from older cells that are born through cell division.