The three substances produced when water molecules are broken down in the light reactions of photosynthesis are:

1. Electrons: These electrons are energized by light and used to power the electron transport chain, ultimately leading to the production of ATP and NADPH.

2. Protons (H+): These protons are released into the thylakoid lumen, creating a proton gradient that drives the synthesis of ATP.

3. Oxygen (O2): This is a byproduct of the water splitting process. Oxygen gas is released into the atmosphere.

Let me know if you'd like more information about how these substances are produced and used in the light reactions.