Final answer:
Colonel Mustard's statement 'Certainly not!' is a denial of involvement in the context presented, showing his disagreement or disassociation with a suggested accusation.
Step-by-step explanation:
Based on the provided excerpt from Scene 3, the correct interpretation of Colonel Mustard's statement, "Certainly not!", is that he is denying involvement. The phrase is a straightforward negation and typically indicates disagreement or refusal. It's clear that Colonel Mustard is responding to an accusation or suggestion he wants to distance himself from, rather than agreeing, confirming, or questioning anything or anyone.