I would think though he is feeling a little lost because he can't "fix" anything. Your other babies needed to be walked, soothed, rocked, sung to, etc...they needed soothing, fixing. This one is content and he is useless to fix anything, there are no problems.

Maybe you can take the other girls out for an hour or so and leave DD3 with him between feeds or something, so he has some time alone where he can find his feet and his place with her without onlookers.