He is the best boy in the class.

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. He is the best boy in the class.
  2. He is the better boy between them.
  3. He is the better boy in the class.
    What about the above sentence?
2

of the three I find the first sentence to be correct

if there is a comparison to be made then better is usually limited to two as in “you are better than me” or “of the two I consider the first to be a better choice”

same goes with sentence three - class refers to a large number

3

He is the better boy between Rahim and Karim.
What about the sentence?

4

That would have to be
'He is the better boy of the two: Rahim and Karim.