AI would say definitely three, if not four.
I would say 4". I have that 5 petal blossom veiner and the flowers come out at just over 1" wide. Those are the white flowers on this cake.
It looks to me like the top tier is off-set so there is probably only 2" difference.
Based on cakes I've done I'd say there was a 4" difference in the tier sizes on that one, which would make for a 2" "ledge" on each tier.
Thanks everyone!!!! :)