Yeah, the icons aren't precise yet, in the sense that they all afford plausible but wrong interpretations. We want to minimize plausible interpretations, ideally there should only be one plausible interpretation, the correct one. Let's look at some wrong interpretations:
Three buttons with not-equal:
Beat the level with
n assignments where no two assignments can be the same skill, in addition to what the other restrictions impose.
Indeed a strong wrong plausible interpretation. Is this why you deem the not-equals sign confusing?
Three buttons:
Beat the level with
n assignments total.
This is the weakest wrong interpretation because the wrong interpretation already exists with a different icon (crosshairs?). This can be an argument that three skill buttons without any symbol is a good icon. The problem with this argument is that the player must know about the crosshairs icon to draw the conclusion that three buttons don't mean
n assignments. Therefore the three buttons by themselves still aren't as precise as we want. I'll have to sleep over it some more.
One button with question-mark:
Beat the level with
n assignments of a single skill of your choice, and this is the only skill allowed other than the explicit skills from the other restrictions.
This sounds really plausible a wrong interpretation to me, even stronger than the three-buttons-with-not-equals. Consider: If you were to implement such a talisman restriction, isn't this the obvious icon for that?
One button can mean:
Vague. Weakly the same as one button with question mark.
-- Simon