I noticed the extra space for more slots but I chose to think nothing of it.
Here's what I'm hoping now:
One slot at start.
I get to Lvl60 with my first character and the other two slots unlock...
or
one of them unlocks and after that one's maxed, I get the third.
If it doesn't work like this, I might not bother with this platinum. If data is server-sided [and I doubt we can copy saves for this game] then the smartest move would probably be to the other two factions you DO NOT want to be a part of first if you care for the platinum... which would tick me the hell off.
If it doesn't work like this or what gaming_best's post stated, I probably won't bother getting platinum for this game.
4 ≈ One