The 3-Year Plan grants a PRO version license valid for 3 years.
Unlike the Monthly Plan and Annual Plan, this plan cannot be renewed. Therefore you cannot continue to use it for a 4th year.
If you wish to continue using the PRO version for a 4th year or beyond, you must buy a new plan at the time when your 3 year license expires. (Please note that you cannot buy multiple licenses in advance for future use.)