Simply add the following line
resource "google_container_cluster" "primary" {
name = "cluster"
location = "us-central1"
initial_node_count = "3"
in order to create a regional cluster. The above will bring up 9 nodes with each zone (f a b) containing 3 nodes. If you only want 1 node per zone, then just change initial_node_count to 1.
More info here at Argument reference.
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