Since the requirement is just for 3 months, then the best cost effective option is to use On-Demand Instances.

Key Points:

• Spot Instances are for fault-tolerant, flexible workloads and can be interrupted
• Reserved Instances (both No Upfront and Partial Upfront) require 1-3 year commitments and are cost-effective for long-term usage
• On-Demand Instances provide maximum flexibility with no long-term commitment, making them ideal for short-term development and test environments
• For a 3-month duration, On-Demand offers the best balance of cost and flexibility without the commitment of Reserved Instances