Coffee and Books

Hi everyone, I'm applying to PhD programs in English for fall 2020 admission. I completed my MA in English in 2018 and I have gotten two professors to write me LORs. However, I'm having trouble finding a third reference. Two other professors from my MA have said no (one couldn't write me a strong letter because I got an A- in her course and the other said she had too many letters to write this semester). I have asked a prof from my undergrad who wrote me a letter for my MA (it was from a fourth year seminar course, if that's helpful) and while she agreed to write me one she said it wouldn't be a strong application decision because admissions committees want to see graduate references. I have asked another professor from my MA but I haven't heard back yet. But if she can't write me the letter, I'm wondering how bad it will be to have an undergrad professor as my third reference.