It was from memory. I distinctly remember it being advertised as "four nines". Perhaps they've adjusted their marketing. The S3 FAQ still says [1]: "S3 Standard is designed for 99.99% availability".
It is 3 nines [1] for availability and .999*365/12/24=1.266 hours per month so it is actually 3-4 times their monthly SLA depending on your get/put requests (gets came up sooner than puts but that is not factored into the SLA AFAIK).
S3 promises four nines of availability (11 nines of durability), so today we got about 3-4 years worth of downtime in one fell swoop. Oops.