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So, question. I have three people writing letters for me. Two of them have been written, but it'll probably be another week until the third is written. One of the two written knows me less than the still-outstanding writer, who knows me very, very well. My question is, if I submit my apps now, will that third letter get to schools once it's written? Will it be considered?

I would submit applications ASAP. It's pretty late in the cycle. The other LoR that's coming can be used to campaign off a waitlist.