Condition if any instances of object at same frame

I have three instances of an object with 30 frames that represent unique images. I set the frame (image) using Random and that works as planned, but I would like to check to see if any of the two objects are using the same frame. I want all three objects to be unique. If they are not I want to regenerate the three until they are.

I am new to dealing with conditional expressions: how might I approach this?

Would it not be better to use Random Pool or Random Multipool for this? You would be guaranteed that all 3 are different on the first go.

As for doing it without any sort of random pool object or technique, it would be very difficult due to the way MMF2's object selection works, and would be very confusing to understand.