Alessa, sorry to hear this. However as I'm a very keen wildlife spotter, birdwatcher and wildlife photographer I've heard an experienced this before. We have to accept that with natural breeding of birds that some chicks may not survive. Sometimes happily the entire brood survives but sometimes, sadly, the whole brood can die before fledging (leaving the nest). This can be caused by many reasons, predators such as magpies and Jays or crows can attack the chicks and kill/scare away the parents. It's common for the predators to drag the nestlings from their nest and until they can fly but once they are badly injured there is almost no hope. Even if the parents stay and defend the nest they can't do much about an injured chick which can't fly. It could also be that these that died were simply nestlings that were part of a brood that had already fledged but those 3 were ill and unable to fly- therefore falling from the nest to thw ground where their injuries would get worse. I noticed the pictures of them in your house, I definitely wouldn't have brought them into the house. I would have put them in an outdoors place such as in an open shed or garage so that they could fly if they wanted to. You made a mistake by feeding them milk and bread, mealworms and small insects would have been the best bet. Obviously you wanted to help, if this happens again at least you'll know what to feed them. You can't use the excuse of
money, it costs nothing to get a few insects from the garden. By all means, if you find chicks on the ground which look ill or are unable to fly then wait a few hours watching from a good distance away (make sure they're not baby Tawny Owl because it is common for them to be found on the ground where their parents DO rescue them). Don't bother calling the RSPCA, they tell you to take it to the vets who nearly always put them down straight away. If you know anyone who has an aviary then this is the best bet for the survival of the birds, although do accept that wild animals suffer extreme shock after being handled therefore the chance of survival is always low. Sometimes the best way is just to leave them where they are. Nature can be a cruel thing.