self-appointed cat expert here...
You are feeding Kitty appropriately. Within a week her kittens will probably start to sample wet food and maybe even dry food, but they will still be nursing for important protein and antibodies. Continue feeding her as much as she wants, and giving her plenty of water, until the kittens are weaned or separated from her. Do not
give her cow's milk, even if she likes it; it will cause diarrhea (the same goes for the kittens.)
I would suggest getting a large box and trying to move Kitty and the kittens more out in the open. Lay the box on its side and put a shallow litter pan (no more than 1/2 inch lip, use a cardboard pan or a disposable cookie sheet) and a couple of towels inside. Move the kittens first and Kitty should follow. Kitty may move the kittens back under the bed, but trying to get them to a place where you can reach them more often, and where they can see and hear you better, will help with socialization. If Kitty moves them back, leave the set-up box so she has the option of moving them back; also as the kittens get more inquisitive and independent, they may choose the box over the bed.
At 3 weeks of age the kittens are just starting to be able to see. They can smell their mother but still get disoriented when picked up. They are used to being right on the floor or next to Kitty, so being picked up by human hands is both unsteady and unfamiliar. Nevertheless, they are old enough to be handled and you should do so regularly.
The only warning I have is that Kitty could become overprotective. What I suggest is that when they cry and she comes over, give her a reassuring pat and let her sniff or groom the kitten while you hold it in your hand. If Kitty grabs the kitten by the neck and carries it away, let her take it. Don't come between her and her kittens if she becomes assertive in that manner.
Kitty's closed-mouth meow is reassurance to the kittens, telling them that she is nearby and watching them. Within a week the kittens will probably start getting curious and wandering farther from the nest, and she will make that noise when one of them gets "lost." Their hearing will be good enough by then that they will be able to turn around and head home.
Within the next week or so, Kitty should start teaching the kittens how to use a litter box. This is instinctive behavior and if they fail to adjust you can help by putting them in the litter box immediately after they finish nursing. Most kittens will learn within a week of the beginning of litter training.
Ideally, you should keep the kittens until Kitty weans them herself. Some very patient mom cats will allow their kittens to suckle long after it's necessary, but most wean their kittens by 8-9 weeks (ouch, those little teeth hurt!) The best indicator that they are ready to be separated, if they continue to suckle, is if they are eating dry food by themselves. Right now you can start making wet food available, but as their teeth come in they should be able to eat dry food alone by 8 weeks. Of course that should be dry kitten chow,
which they should eat until they are one year old.
Here's the way to "sex" the kittens:
Males have one hole under the tail and a little "round mound" about 1/8 to 1/4 inch below that.
Females have a hole under the tail and a little slit below there.
This is not a firm rule, but only based on the picture:
It is more likely that the red kittens are male. The brown kittens could be either male or female. If any of the brown kittens have a patch of red on them, they are also probably female. But coat color does not
If you have more questions feel free to e-mail me.