LOR Advice

[Statman4]

I initially posted this in the wrong place(I'm a noob here).  Sorry if it shows up twice.  I'm in a bit of a precarious situation with respect to my LORs and I was hoping for some advice.  I finished my undergrad 10 years ago and I'm currently in the process of applying to grad school for Masters in Statistics.  I have taken some course work as prerequisites for grad programs(CalcI-III and Linear Algebra).  I'm getting two strong LORs from tenured professors from these courses but I am unsure what my best option is for letter number three.  I have three potential professors I could ask but they seemingly all have drawbacks. 

1. A calc professor- I got an A+ in his class.  On several occasions he made it clear that he considers me to be an exceptional student.  The drawbacks as I see them are that he is an adjunct and seemed disinterested when I asked for a LOR.  I believe that he is a retired engineer and teaches to stay busy.   I asked him via email if he would write a letter and he agreed.  In my email I offered to come meet with him in person and provide him with my transcript from my undergrad institution or a CV so he could personalize the letter.  He seemed disinterested in his response which was one sentence, something along the lines of "ok where should i send them".  I took the class last spring so there is a possibility that he doesn't remember me.  English isn't his first language so its also possible that what comes off as being disinterested is really just a language barrier/cultural differences.  I'm just concerned that he will write a lukewarm, impersonal letter which from my understanding will do far more harm than good.  

2. Another Calc Adjunct.- I got an A in his class but he has only been teaching for a year or two.  I have a decent relationship with him but I don't think I stood out as much in his class.

3.  A tenured professor from my undergraduate institution- I worked closely with her as a research assistant.  I'm mentioned in the "Acknowledgment" section of at least one of her papers.  This would obviously be the best option if it wasn't for the fact that I haven't been in contact with her for ten years.  I feel like admissions departments won't be blown away by a letter saying I was great to work with in 2010.  

I feel like all my options are sub-optimal.  Any thoughts or advice would be greatly appreciated.  

Edit: spelling

Edited February 20, 2020 by Statman4