First, fast her for a day and then feed her some green de-shelled peas (the type you buy frozen but take of the skin). If you can't find these, get some daphnia. These things will help if she's constipated.
Is she bloated-looking? Are her scales sticking out? These would be dropsy (a symptom that results from organ failure and accumulation of fluids in the body). This can rarely be treated successfuly but three-spots are hardy gouramies so it's worth a shot. This is normaly brought on by internal bacterial infections and requires the use of antibiotics. You need to make sure you follow the instructions on the bottle precisely and, even if she doesn't pull through, continue the treatment as instructed. If you can isolate her to a CYCLED hopsital tank, it is worth doing so as she is the only fish affected and treatment can be both stressful and cause problems with biological filtration (so make sure you monitor water parameters and do frequent water changes as necessary).
Again, however, don't try this until after fasting her. Honestly, once a gourami stops eating, it's unlikely to get better and you may want to consider euthanasia (put her in a bowl of tank water and add some clove oil - you can buy it at most pharmacies for toothache). It would be wortht empting her with some live foods (eg blackworm) after you attempt the daphnia/peas to see if she'll still take something.
Stringy, white poo can also be a sign of parasitic infection though this is quite unusual in the majority of aquarium fish. You can get hold of various meds. for this but you may need to have them prescribed by a vet. My experience here is limmited as I've never needed to use them. Salt (AQUARIUM salt) can also sometimes help with parasites - at least to prevent it from being spread to tankmates. There are probably pinned articles in the emergency forum on using salt. Having said that, parasites don't usualy cause a loss of appetite and they shouldn't cause bloating. In fact, they tend to cause a gradual 'wasting away' instead. The stringy poo can be explained otherwise by the fact that she's not eating.