C3PO is one assured evade. The roll is irrelevant. C3PO just improves the roll you choose from 37,5% to 100%.
C-3P0 is only an "assured evade" because with one die the probability of rolling an evade and the probability of getting one from C-3P0 happen to add up to 1, purely by coincidence. Look at it this way:
Roll an evade on the green die without C-3P0 = 1 evade.
Roll an evade on the green die with C-3P0 = 1 evade.
Since the outcomes are exactly the same then it doesn't matter whether you had C-3P0 on that roll. And I'd say "having C-3P0 and not having C-3P0 produce the same result" is a pretty good definition for "C-3P0 did nothing that turn".