If you have TRICARE, you must have Medicare Part B to keep this coverage. However, if you are an active duty service member, or the spouse or dependent child of an active duty service member, you may not have to get Medicare Part B right away. You can get Part B during a Special Enrollment Period, and in most cases you won't have to pay a late enrollment penalty.
(Primary Source: Centers for Medicare and Medicaid Services - Medicare and You Handbook. This content may have been enhanced by Q1Group LLC to include further examples, explanations, and links.)
Medicare Supplements fill the gaps in your Original Medicare