Help Me Save Briar, Beau, and Blaze

Hi guys, my name's Cebastian and the girl up top is Briar. She's a maltipoo, she's only 3 months old, and she's fighting off a really bad sickness right now. We haven't gotten a diagnosis yet, but we already took her to the vet for bloodwork, x-rays, and fecal tests. We think it's worms, a parasite, an intestinal issue, or a combination of any of those. She's lethargic, isn't eating, has liquid diarrhea, with occasional vomiting. She has two siblings, Beau and Blaze, who are both now showing signs of lethargy, so whatever she has seems contagious. We've already spent close to $1,500 in the past 4 days alone with Briar seeing the ER and her vet. We want to take her brothers in ASAP but we don't have the money for anymore examinations, tests, or medicine. I don't want my puppies to leave me so soon. Please help me care for them.