If you are an active employee who becomes eligible for Medicare Part B while covered by RF health insurance, you should not enroll for Part B until three months before you retire.
Since you'll have continuous coverage with the RF as an active employee, you won't be penalized by Medicare for joining the program late.
You must, however, enroll for Part B before you retire to avoid a late enrollment penalty and possible reduction in your RF benefits.
You should not enroll in Medicare Part D, even if you are eligible to continue health coverage as a retiree, because the RF provides better drug coverage and you cannot use both plans.
Refer to the Continuing Benefits section of the Benefits Handbook (How Medicare affects your health insurance benefits).