The previous educator answers are correct. To distinguish one person, place, or object out of three or more, you would always use the superlative form. Thus, the question should read as follows:

Who is the most talented of the three girls?

As both previous educators have noted, the comparative form is used to compare two people, places, or things.

Both comparative and superlative sentences use the sentence's adjective as the point of distinction. In this case, the adjective is the past participle "talented." Most comparative adjectives that are one or two syllables will take the "-er" ending. Those that are three or more syllables will use the modifier "more" before the adjective. There are, however, some exceptions, such as adjectives that end in "-ish," such as "stylish." The same rule occurs with superlative adjectives. The adjective takes the "-est" ending if it has one or two syllables and the modifier "most" if the adjective is three or more syllables. "Talented" has three syllables.

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.