Answer
About 5.4 minutes per month. In review, anything from 3.6 to 8.1 counts as right.
The worked estimate
All three must be down at once: 0.05³ = 0.000125, so availability is 99.9875%. A month has 43,200 minutes: 43,200 × 0.000125 ≈ 5.4 minutes. Three mediocre replicas beat one excellent server, as long as their failures really are independent.
Numbers: Numbers to know.