I have asked this question, and one of the first comments suggested that it was developer intent/required developer knowledge.
I could be persuaded either way, so at what point does this kind of question become "developer intent"?
It would seem perfectly acceptable to me to ask what the third slot is for. It is a thing that is in the game and you are curious about it.
(Full disclosure; I am basing the following as someone who has not played Destiny, and is relying on your description of your question and the link in the question to provide context)
But it sounds like whatever goes in that third slot is not yet implemented yet - which means the only possible answer for the moment is "we don't know". Anything further would be speculation of developer intent, or speculation on unreleased content.
We can not know what goes into that slot until whatever it is has been implemented. Any plans the developers had or have for that slot could change between now and whenever something is released to the public.
Which brings us to the probable reason why your question could conceivably be considered off-topic as it stands: It takes speculation on the likely purpose of the slot, and then (partially) asks how the speculated system would work. If there is any doubt that this is what you are essentially asking, you yourself follow it up with "Or even what it is actually for?".