What % is 200g? I'm afraid I can only work in old money and you also need to include birth weight for comparison. Is she now gaining weight or still losing/not gaining? How are other things like weeing, pooing? How often is she taking a feed? How many sides per feed? Night feeds? Evidence of milk transfer? How are your breasts? How does she behave during a feed?
Are you still seeing the midwife? Do you have a local breastfeeding support group, counsellor, peer support, hospital services etc?
It is of note that a 3wo is still not up to birth weight. If bfing is going well, you would expect it to have happened by now. It is important that the situation is monitored/ameliorated. But have you had no advice whatsoever about increasing your milk supply and/or milk transferece? Has somebody who knows what they're doing observed an entire feed?
For now, I will recommend a few things to increase supply and milk transfer (which should have been recommended by your hcps already but sadly, it doesn't happen). Then hopefully somebody like tiktok will be along (maybe while I'm typing) and they'll have better advice I'm sure.
Lots of skin to skin. Feed as much as possible. Offer both sides and then a third, fourth etc side as many as she will take. Keep her close to the breast, respond to each cue and if she doesn't ask, offer anyway. Look at the kellymom site, find out about breast compressions, look at jack newman's site and videos.