A. Percula and A. Ocellaris are generally some of the more docile clowns. There are examples of them working with more than 2 but it is normally ill advised. For one if you do go this route it would be a good plan to get them as juveniles, three already sexed fish will fight almost for sure. The smallest one will still most likely get picked on though.
As for having three clowns with only one anemone, I would not reccomend it, simply because there is not enough territory for them, even if the anemone is large the clown pair will claim the whole anemone as theirs and will not want the "third wheel" in the anemone with them. I would also reccomend against an S. Haddoni becaues not only are they hard to keep they are known fish eaters (I my self have lost two puffer fish and a trigger to a large Haddoni). They have one of the most potent stings and have occasionally eaten a. ocellaris and a. percula clownfish (it is not their natural host).
S. Gigantea while still very hard to care for is a natural host, if you are stuck with going with a carpet anemone.
There are some examples where more than 2 clownfish work, these are shown by members such as Mobert who kept many clowns together, but these clowns have numerous anemones and are not in competition for them.
I have been fortunate enough to be able to keep 3 clownfish together, 1 A. Perucla/A. Ocellaris pair, and a third A. Ocellaris. But I also have multiple anemone for them to go in. The Pair stick to the Magnifica and the snowflake keeps to the BTA's, they spawn and still leave the snowflake alone in his own nems, despite the fact he is only a couple inches away from the clutch of eggs.
.... Just sayin
