The rule of thumb is one nest for every 4 or 5 hens. If they are 12" x 12', maybe you need another nest. If they are around 16" x 16" those three are probably enough.
You should not be getting cracked eggs due to activity. That sometimes shows the shells are too thin and they need more calcium in their diet. How are you feeding them? How are you supplying calcium? If all they eat is a Layer feed they should be getting enough calcium but if they forage for much of the food or you feed them low-calcium "treats" offering oyster shell on the side may be a good idea. I'd offer it on the side anyway.
How well padded are your nests? You need enough bedding to cushion the eggs when they are laid. What do those cracks look like? Could you post photos of the cracked eggs and the nests. That might help us determine why they are cracking.