Baby #3 has been a "grounder".
This is most likely Baby #1 judging by its behavior.
Baby #3, alas, is totally recognizable. This baby broke its ankle joint. (I don't know the correct term.) Both of its legs were fine while a nestling. I've looked carefully at the pictures, especially when it was standing alone on the edge of the nest. Something serious happened after it left the nest. The parents are still feeding it. It scoots across the ground, but it can't get lift-off to fly up to a branch. That may change in a day or two as it develops more strength. I have seen robins with this malady who have reached adulthood, but this doesn't look good.