Final answer:
The third main component of all cells, alongside the plasma membrane and nucleus, is the cytoplasm. This gel-like substance houses various cell organelles and is key to cellular function.
Step-by-step explanation:
The three main components of all cells include the plasma membrane, the nucleus, and the cytoplasm. The plasma membrane is a crucial cell structure that acts as a barrier, maintaining the cell's environment by regulating the movement of substances. The nucleus contains the cell's DNA and manages most of the cell's activities. Lastly, the cytoplasm is a jelly-like substance where cellular components are suspended, and it contains the cell's cytosol and organelles, such as the ribosomes, which are involved in protein synthesis.