Just use -var 'az=0'. That's it.
The EL2 modulo distribution feature enables you to use just a single launch invocation to spread out instances across Availability Zones(AZs) with the round robin like property of modulo arithmetic; resulting in an as even as possible instance distribution and making it far simpler to use the most fundamental resiliency feature of the AWS offering. If there are 3 AZs in a given AWS Region and your launch invocation requests 6 instances, specifying -var 'az=0' will result in 2 instances in each AZ (assuming the launch is successful). Similarly a request of 9 instances, would result in 3 instances in each AZ. A request of 5 instances would result in an instance distribution of 2, 2 and 1 across each of AZs 'a', 'b' and 'c', respectively. To target just AZ 'a' for all instances, specify -var 'az=1', or for AZ 'b' use a value of 2, and so on.
Here's how a full command line looks for a modulo distribution of instances in Europe (London):
terraform apply -var 'region=eu-west-2' -var 'access_key=...' -var 'secret_key=...' -var 'instance_type=t4g.nano' -var 'instances=3' -var 'az=0' -var 'linux_distro=debian9stretch' -auto-approve
