At only 3 weeks since mating she should really not have gained any weight. If she becomes overweight this will make whelping more difficult.
She should be kept on normal rations until it is confirmed she i9s in whelp at about 30 days (I have mine scanned) then you can switch to a puppy food, but keep the quantity the same until the 5th week. Gradully increase the amount fed until she is eating half as much again asnormal by whelping.
With my breed this would mean 200g up to 5th week, then 225 in fifth week, 250 6th week, 275 7th week, and 300g in last week.
Once pups are born then I feed as much as the bitch wants until pups are being weaned. I graduly cut the bitches food down when she feeds them less and less. I keep them on half as much as normal rations of puppy food until they loose their coats and regrow it, and regain their figures, adjusting if I think they are getting fat.