I am in the same situation. My third letter writer is the one who I thikn would write the best letter for me, but he is super flaky lately and I am tired of chasing it. I just don't know what will happen because I listed him as a recommender on my apps, and I am afraid my otherwise complete files will sit and wait for his letter. I need to put some time into research on this issue... sigh.
I was worried about this too (except for my 4th rec, not my 3rd).. until I actually applied.When you go to add schools under the LOR tab on LSAC... in a little box it tells you each schools policy. For most of the schools I applied to (look at my sig) it said they would accept a new report as soon as a new letter came in. A couple of schools only get updated reports at a certain time each month, and I think one or two said they do not take updated reports (meaning your SOL for those ones).But, I'm fairly sure for most of the schools, you will go complete as soon as you have the minimum required number of LOR, but they will accept more as they come along, up to the particular school's limit.Additionally, I can't remember which school, but one application gave me the option of holding my completion until all recs were in.Does that help?
Same thing is happening to me (see my thread "Help! One of my letter writers is ignoring me"). I just had the 2 that I have sent in. I then notified all the schools that asked on their applications how many rec's I'd be sending or who would be writing recs that to consider my application complete once the two from recommender's x and y came in. I also let them know that a 3rd rec from recommender z may come in later, but not to wait for it.