The much reduced shorter upgrade times helped players level up quicker - the instants were just a nice bonus for select buildings - neither of which have any bearing on whether we need a third slot.
If we did get a third one, why stop there? BHG might as well give us 6 slots, make all upgrades max at 4days, have all legendaries cost 10 tokens - available each week, - and have all councillor packs give purples.
Might as well make this game as easy as candy crush.