1 in AMS2;
1 in SGP1 and;
2 in NY2.
Each cluster contains (upgrade as needed):
1 Caching load balancer (2 cores, 4GB of RAM)
2 web servers (2 cores, 2GB of RAM each)
1 MySQL server (2 cores, 2GB of RAM)
Use private networking to let the droplets communicate with other droplets in the cluster.
Decide on a cluster that you want to push edits to and use rsync to keep the other clusters synced and are mirrors of eachother.
For DNS, use something like Dyn or Route53 to route traffic to the closest cluster and enable failover if a cluster goes offline.