Oh, kittens! I am so happy you all decided to let her come inside! Now, the part about there first being 7 kittens and now only 3 concerns me a bit...can you clarify that? Did you ever find the other 4 or did I miss something?
I don't recommend milk for cats, but the suggestion that Sandie left to make sure that mom is eating kitten food is a very good one. Changing food or adding milk will sometimes upset delicate tummies, so you might want to bring about the change slowly, mix a bit in with her regular food until she's eating all kitten food. You can try adding a small amount of milk to the dry kitten food for her, but back off if she starts to vomit or have loose stools.
Signs of infection would be: Off eating or drinking, litter box habits after a delivery can sometimes be off, so that might not be a good indicator, but if she is using the box, check for anything unusual there, if you see any abnormal discharge, or if she feels hot and isn't interested in caring for the kittens.
By all means, the more human contact the kittens have, the better. Keep them in a room with mom by themselves if possible, and make sure there aren't any good hiding places available for mom to spirit them off from the evil monsters in your home *grin*. Go in and sit with them as often and for as long as you can. Start to put your hands on the kittens as soon as you can - this is called "imprinting". The more you handle them, the more socialized they become.
We would love to see pictures if you have them, or can send them to me and I'll post them here if you like.
Please keep us updated - we are here to help.
My continued best to you,