Who can be my third reference for doctoral applications?
Don't be shy. While this advice works in a million context, it is especially relevant here. The only way to know if you will get a good letter, is to interact closely with senior people in the field, gauge how much they liked your participation in their classes/your work in their lab, ask them the letter, gauge their response, choose the one that answers more enthusiastically. Your first two choices were the most obvious, now you should try with other lecturers you had. You certainly did not have just one lecturer in your studies! 

I was assistant professor in a research institution in my previous job, and can see several reasons why a lecturer might decide to write or not a letter for a candidate. You will never know them, you just have to ask, and use your social skills to predict whether it will be a good one.