If 3 molecules of glucose went through glycolysis and into the citric acid cycle, how many times would the preparatory reaction occur?

6 times

Explanation:

The process if cellular respiration is divided into three major phases : Glycolysis, Kreb cycle and ETC (electron transport chain). Starting from Glycolysis, which occurs in cytosol and breaks the simple glucose molecules. It is the first step of the cellular respiration, in which one glucose molecule is broken down into two molecules of 3-carbon pyruvates.

Each of these 3 carbon pyruvate molecules lose a carbon to form acetyl-Co A and enter into Citric acid cycle- the second stage of cellular respiration. For each pyruvate going into citric acid cycle 1 GTP , 3 NADH , 1 FADH2,  and 2CO2 molecules are formed. The stage is next followed by electron transport chain in which greater amount of energy in the form of ATP is synthesized.

So, we have seen that each glucose molecule starts two preparatory reactions because 2 pyruvate molecules are formed and each enters into separate citric acid cycle. So, if there are 3 molecules of glucose 6 preparatory reactions separately will occur.

Hope it help!