Letter of recommendation advice for Biostat PhD

[bstat95]

Hi everyone,

I'm applying for Fall 2021 Biostat PhD. The letter of recommendation thing has literally stressed/depressed me for months, so I'm here looking for advice on my 3rd letter.

I have two letter writers so far. One will be my masters thesis advisor who is fairly well-known (h-index>65). I'm currently pursuing a masters in biostat at one of Harvard/JHU/UW and have been doing research with this prof for a while. The letter should be reasonably strong, hopefully.
The other will be a research fellow/biostat & math prof I worked with for 1 year at the national research institution of my home country during undergrad. I did absolutely nothing meaningful there. But the prof speaks very highly of me and actually let me look at the letter, which I think it's pretty strong.

Now I'm trying to decide who to ask for my 3rd letter...

1. A prof from my undergrad institution whom I took graduate level statistics with. She wrote one of my letters when I applied to masters. I was frequently top of the class. My biggest concern is that the prof doesn't know me well, so it must be a very standard "do well in class" letter. Also, I'm afraid this letter would be basically useless since I'm applying with a masters.
2. My undergrad academic advisor (a public health prof). Also wrote one of my letters when I applied to masters. He knows me very well in terms of my career plans and academic performance in undergrad, and he seems to like me. But he can't speak much, if any, about my math ability or research potential in biostat.
3. A prof from my undergrad institution whom I took linear algebra with. I'm hoping if the prof can write about my math preparations because although it's an undergrad course in linear algebra, this course was extremely difficult and proof-intensive (math at my undergrad institution is generally very hard) - it's actually much harder than any upper level math courses I've seen in my current institution. However, the prof doesn't know me well, and I only got an A while 10~15% of the students got A+.

I've been feeling SO bad about not having built a relationship with professors (especially professors at my current institution), and worried about not having strong letters to boost my application. Any advice is greatly appreciated!

[bstat95]

This is my profile:

 

[bayessays]

I think your grades will speak for themselves in terms of being qualified to do the math. I would probably go with letter 2 as they know you the best and like you, though an argument could be made for letter 1 as that is a more quantitative professor.  Letter 3 does not sound like a good option at all.

I would not stress though, because you will be in very good shape having two letters from professors at a top department, so the third letter will just be icing on the cake.

[cyberwulf]

The letter from your Masters advisor will carry by far the most weight; the others are (relatively) less important. Seems like letter writer #2 would be your best option.