As many others have already said, it depends on the nature of the issue. I'll try to provide a serious response to the question.
My first priority, especially in such a short period of time, would be to apply containment. What can you do to protect the customer i.e. prevent the issue from being delivered into the field? This can be utilising the next batch of materials (if you have evidence it's batch-specific), performing added inspection, rework or some other workaround while the root of the problem is being investigated and a proper corrective action can be applied.
Root cause analysis and corrective action can take much longer than 3 hours depending on the nature of the issue so containment can buy you time.
Then there are various root cause analysis tools available such as 5 Whys, Ishikawa, or KT. I would say finding a root cause is usually the hardest part in problem solving. In saying that, don't forget about the other important aspects such as:
Problem Statement, Impact/Risk Assessment, Containment, Disposition, Corrective/Preventive Action and Effectiveness Check.
Good luck my friend, I hope you didn't get fired already.