I would not just give her a bath, I would completely let her soak in it. If she's got organ failure then there's nothing you can do to save her, but you are correct, it will make her a little more comfortable. I would use a 3 teaspoon per gallon of Epsom salt dosage for Dropsy cases. Melt the salt in a different cup using her tank water, either acclimate her slowly to the new water or if she's already in the tank then slowly add the salt to her tank over forty-five minutes since the dosage is higher than usual for normal things.
Daily change her water, 50% will be fine for 3 gallons, under 3 gallons do a whole 100%. Remix your 3 tsp/gal and acclimate her over half an hour or so.