it is ok, if one of the letters is not written by a Prof?
I have these 2 Prof writing my a letter of rec:
A: Done research in his group, write 1 paper as co-author, 2 talks
So he could write something research-related.
B: All my theoretical physics courses were held by him and I will write my bachelor thesis under his supervision. He described me as a "excellent student" in front of other Profs, so this should be a strong letter, too.
But now the problem starts with the third letter:
C: Math-Prof, had him 2 years in Calculus... was the best in his class. But that's nearly all he can say.
D: He is a PhD student. But I have done all my research (for Prof. A) under his direct supervision. So he knows me since 2 years ago (actually, he is the one who knows me the best) and could give some insights into my potential for research. Additionally, his letter maybe will duplicate some points of the letter of Prof. A (is that good or bad?).
So C or D or (C and D)?