
By the end of the third week, going into the fourth, you should be starting the weaning process - and she knows this. Some bitches will go as far as to regurgitate their food for their puppies, feeding, going into them and doing this on the floor of the box. It's gross, but natural.
She's telling you now is the time to introduce them to food, starting with goats milk, warmed, 4 times a day - holding each puppy individually and encouraging it to lap. Some get this faster than others. Once they can all do this (and give what's left over at each feed to mum), then you add a good quality puppy food, mixing it with either the goats milk, or just warmed water into a porridge. And again help each puppy to tuck in. Once they all can, then you fill a shallow dish and put it on the floor of the box, with mum out of there and with any luck, they'll pile in. Once they have finished, then let mum back in to clean up!! And she will probably allow comfort suckling, standing up BUT as soon as she feels the needle-sharp teeth, she will want out again. And this, the teeth, is why she's not willing to hang around with them any longer.
We'd move our litters from the quiet living room where they spent the first 3 weeks or so, into the kitchen, adding another section to the existing whelping box for more space. And as most of our litters were born into the better weather, they'd all spend increasing amounts of time outside in the fresh air. A bit of sun on their backs brought them along by leaps and bounds.
Mum doesn't need to be in with them all the time, and should be completely off them by 6 weeks - all part of the weaning process and necessary so they go to their new homes at 8 weeks, or older, only missing their siblings and surroundings, not mum.
If she's still producing a lot of milk, cut back on her food, especially the protein. Allowing comfort suckling helps her to offload some of the milk, but nature usually helps her to know they don't need what they have been taking from her, so her milk is scattered naturally.