Anytime you are wondering about a question of this nature, think about your English teacher's "compare and contrast" questions: If you are to compare something with something else, that means that there inevitably will be two. For a visual, consider the Venn Diagram: one thing on one side, one on the other, similarities in the middle. Comparative, from the same root as compare, means two things are being examined.
Sue is more talented than Anne, but Betty is the most talented of all the girls.
Concluding, if you have two of something, use comparatives (more, etc.). If you have three or more, use superlatives (most).
You would use the superlative form here:
Who is the most talented of the three girls?
The comparative adjective is used in regard to two people or things. The superlative is used for more than two.