Congrats on your new baby! Yay!
As for the 5k, what about a compromise? You could put half on student loan #3 (higher interest rate than car) and half toward saving for your next kid.
I'm not sure of your ages or your desired age gap between kids, but sometimes having a kid takes longer than you think so waiting a long time to start trying can leave quite a large age gap. This means it will also be longer before they're in school and she goes back to work.
There are no right or wrong answers, just what the two of you feel works best for your family.