If you're gonna be buying supplies with resources, I'd suggest a conVersion setup something like this:
- 1 of any single resource type = 1 supply
- 1 of two different resource types = 3 supply
- 1 of all three resource types = 6 supply
This encourages you to use all three resources, but if you happen to have a stockpile of one specific type, you can convert just that resource into supply.
It goes without saying that extra resources that exceed your storage should automatically be converted into supply for you at the most favorable rate (e.g., an extra 100 minerals and 50 organics would yield 200 supply).