If you are enrolled in Medicare Part A and Medicare Part B and you also have TRICARE for Life, you almost certainly do not need any other supplemental health insurance. TRICARE for Life[military.com] is a special program specifically designed for people who are also enrolled in Medicare. This TRICARE coverage works just like a separate Medigap supplemental insurance policy. That is, for any medical care covered by Medicare Part A or Part B, Medicare pays its portion of the bill first, then TRICARE pays the part of the bill Medicare does not pay. The only hole in this arrangement is for a few medical services that Medicare Part B covers but TRICARE doesn't, such as chiropractic. For those few services, TRICARE doesn't pick up the portion of the bills that Medicare doesn't pay.
TRICARE for Life also provides coverage for prescription drugs[military.com], so you don't need to enroll in a Medicare Part D prescription drug plan, either. If at some point you have very low income and few assets and so would qualify for free or nearly-free Medicare Part D coverage, you can enroll then in a Medicare Part D plan without paying any penalty for having waited past age 65.