I believe the lowest ilvl that drops in inferno is like ilvl 51, so any legendary below that won't drop unless you're in normal / nm / hell.
As far as all of inferno legendaries ilvl 51 above will drop in any act. Farming act 3 only means the % chance for the highest ilvl (63) to drop is higher than in act 2, but can still drop in act 2 or act 1.
Thank you so much, I dont know why my friend think every acts have a table loot like d2 ?
So after XX runs of act3 without any legendary found I should not fear to farm lower act... I cannot be more unlucky