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?