Option 1: Fructose is a sugar molecule and is not the substrate of photorespiration.

Option 2: Pyruvic acid is an intermediate product of glycolysis and is not directly involved in photorespiration.

Option 3: Glycolate is indeed the substrate of photorespiration. During the process of photorespiration, the enzyme Rubisco catalyzes the reaction between oxygen and ribulose-1,5-bisphosphate (RuBP), producing phosphoglycolate. This phosphoglycolate is then converted to glycolate, which enters the peroxisome and mitochondria, undergoing a series of reactions to form glycine and serine.

Option 4: Glucose is not directly involved in the process of photorespiration.

Therefore, the correct answer is option 3: Glycolate is the substrate of photorespiration.