The Mishna (Ketubot 2:10, 28a) states that there are certain people who are trusted to deliver testimony that relates to something that they witnessed as minors. Included within that list are those who testify concerning the handwriting of their father, their rabbi and their brother.
The gemara (Ketubot 28a) asks why it is necessary to teach all three (father, rabbi, brother), and resolves that if it only mentioned one's father we would not be able to deduce one's rabbi, that if it only mentioned one's rabbi we would not be able to deduce one's father, and that even if it mentioned one's father and one's rabbi we would still not be able to deduce one's brother.
The problem is that, while one's father and one's brother have aspects to them that mean they need to be taught independently of one another, that which allows one to testify concerning his brother's handwriting should go for the other two as well!
Specifically, the reason that one can be trusted to recognise his father's handwriting is that he was commonly found around his father as a child, which is not the case with one's rabbi, and the reason that he is trusted to recognise his rabbi's handwriting is that he was in awe of him, which is not the case with one's father. But the reason that he is trusted to recognise his brother's handwriting is that testifying to somebody's handwriting is only a rabbinic obligation, and so we can be lenient!
I understand why you cannot deduce the third scenario from the first two, but it seems to me that you can deduce the first two from the third. Teach the case of the brother then - why do you need to also teach the case of one's father and one's rabbi!?