Interesting question, it boils down to whether the 33% penalty is multiplicative ((4/3)^n) or additive like almost everything in civ: (n+2)/3, with n being the number of cities in your empire.Are you sure that three cities, (other than puppets) is the sweet spot?
The capital with palace, stonehenge and (later) hermitage should generate about 1.5-2.5 times the culture of every other city. This factor might get even smaller late in the game, since herm.+const.+broadcast tower are all additive.
If multiplicative, optimum citycount is 1 for quite some time, then 2-4, depending on revenue from allies and happiness.
If additive, citycount hardly matters at all:
4th city vs. 3rd is 6/5, so 20% penalty. More or less equal to what a 4th city can produce.
9th city: 11/10 -> 10% penalty
99th city: 101/100 -> 1% penalty