It is perfectly normal for a hen to eat the eggshell and anything left inside it after a chick has hatched, especially since she has been pretty much starving herself for the past three weeks. It could be that the chick hatched (or started to) but then died - and in the natural world it is a shame to let such a good source of protein go to waste! Not a pleasant thought from a human point of view, but perfectly logical for an animal.
The fact that she has accepted the live chicks leads me to believe that the chick probably died of natural causes - it is rare (although possible) for a hen to attack her own chicks.
I would leave the other eggs under her for another 24 hours or so - she will be happy to sit on the nest for at least this long with the new chicks under her, and it will give the eggs a chance if they are late hatchers. If nothing is happening after this time then chances are that they are dead, but you could always check by floating them in a jug of water at around 37°C. (The temperature is critical, because if it is too cold or too hot it can shock the chick inside the shell and kill it). Watch the egg for a moment - if it just floats then it is dead, but if it bobs around then there is a live chick in there, moving around.
Fingers crossed for the other eggs, but if you have two live chicks already from a compromised hatch then you're not doing too badly.