What advantage is there to placing three sports of leaf extract on the chromatography paper instead of one.?

Answer 1

Well, what happens if you just put one there? You don't know if the results are correct or not. If you put three there, then you will have three sets of results, just in case one is wrong.
Science experiments need to have more than one subject. The more subjects (in this case dots of leaf pigment), the better the results.