What is the MOST cost-effective instance purchasing option to meet these requirements?

Explanations:

Compute Savings Plans provide savings on usage but do not guarantee capacity or meet the requirement of not being interruptible. They are best suited for flexible workloads that can be scheduled based on price.

On-Demand Instances are billed per second with no long-term commitment, making them ideal for workloads with variable usage that cannot be interrupted. Given that the instances run for only 3 hours each week, this option offers the most flexibility without any upfront cost.

Convertible Reserved Instances require a commitment to a specific instance type over a longer period (typically 1 or 3 years) and are not designed for short-term, inflexible workloads like the one described. They are suitable for predictable workloads where long-term pricing is beneficial.

Spot Instances allow users to bid on spare EC2 capacity and can be interrupted if the spot price exceeds the bid. Since the requirement states that the instances cannot be interrupted, this option does not meet the criteria.