I personally would solve this issue with a local module (very easy to set up, just a subfolder with a main.tf that defines the resources[0] then it's called as here[1]) with two input variables[2]: a required ami variable, and an optional count variable which has a default of 3. I hope that helps.

[0] https://www.terraform.io/docs/modules/index.html

[1] https://www.terraform.io/docs/modules/sources.html#local-pat...

[2] https://www.terraform.io/docs/configuration/variables.html