Why not just ask the client what they meant?
Yes, I asked what they meant by "3-tier" database but i got replied with the wiki page on multitier architecture[1]. Which is what i am familiar with.
I am really confused with this term, never heard of "3-tier database". Requirements you mentioned match to what the client is expecting (probably 99.9% uptime, up to 1sec latency, things like that). I can't get the work back now but for the sake of clearing confusion I have, I would like to know about "tier 3 database". Could you please share some ideas on what makes a database "tier-3"?