A company runs a stable production workload on EC2 that will require compute for the next three years. The team currently uses a mix of M5 instances but expects to migrate portions of the workload to Fargate and possibly switch to newer instance families as they become available. They want the lowest possible commitment-based discount while retaining maximum flexibility to change instance families, sizes, and even move to Fargate without losing their discount. Which purchasing option best meets these requirements?