My first pick would child #2 in the sense that come Sept your youngest in care will be 18 months meaning walking, self feeding, responding to some commands etc. so not really a baby anymore.

The older child could be a playmate but you have enough 2 year olds that will be closing in on 3 that that won't be an issue and the older one may appreciate being "in charge" for a change while other 3 year old is at school especially if the older one liked to direct the play sometimes. It also puts too many at the same level meaning you lose them all at the same time. As for the not napping it is easier to find things for one to do then have to supervise a group and intervene, stop them from talking etc. If she is borderline for sleeping on the couch with a movie or better yet on a mat somewhere else with a DVD player and laying down on her tummy to watch she just might fall asleep some days and if not you know where she is for an hour to do your own thing.

The third child fits in age wise but not necessarily without baggage tagging along in the sense of a parent that needs tending and considering you have options do you really want to take on that extra "child" as well as the 2 year old. It could really upset the flow you have now. Also tossing another child into a group the same age might sound easy but often it is the source of conflict since the other children have grown together and worked out the balance of power. Just where the new child fits in, who they play with the most, who gets left aside as new alliances form could make for some uncomfortable days.......

or you let your group that functions so well just continue on their course and sit back with a baby that you will train into the group.