How do you update a single status based on the "worst" of three sub statuses?

[Randal]

I would like an overall Status to reflect the worst of three sub statuses.

Example1

SubStatus1 = Green

SubStatus2 = Green

Substatus3 = Red

Overall Status should be Red

Example2

SubStatus1 = Yellow

SubStatus2 = Yellow

Substatus3 = Green

Overall Status should be Yellow

thanks for your help