To solve this question, we need to analyze the statements given by the suspects and determine who is telling the truth.
Let's go through each statement:
Robert: "One of the other two stole it. I did not do it."
Scott: "One of the other two stole it. I did not do it."
Tommy: "One of the other two stole it. I did not do it."
From their statements, we can conclude that each suspect is blaming the other two and claiming their own innocence. However, since the police later found out that Tommy was lying and there was only one thief, we can conclude that Tommy is the thief.
Therefore, the correct answer is C. Tommy.