A company is designing a numbering plan for its new VoIP system, which will support 500 users across three different departments: Sales, Support, and Administration. The company wants to allocate a unique range of extensions for each department while ensuring that the numbering plan is scalable for future growth. If the Sales department requires 200 extensions, the Support department needs 150 extensions, and the Administration department requires 150 extensions, what is the minimum number of digits required for the extension numbers to accommodate all users, including a buffer for future expansion of 50% in each department?