What are three similarities between photosynthesis and respiration?

Ellular Respiration and Photosynthesis both have an ATP Synthase. These processes both have buildups of H+ and the ATP Synthase transports the hydrogen ions down the concentration gradient. This process is called Chemiosmosis.
Similarities between Photosynthesis and Cellular Respiration.
Cellular Respiration and Photosynthesis are both metabolic pathways. This means that the products created in Cellular Respiration are the reactants in Photosynthesis. While the products created in Photosynthesis are the reactants in Cellular Respiration. They are also metabolic pathways within themselves. In Photosynthesis, the products from the first phase (NADPH and ATP) are used in the second phase of Photosynthesis as a source of energy. In Cellular Respiration some products of glycolysis, intermediate step, Krebs Cycle are the reactants in Oxidation Phosphorylation.