My 3-month-old Labrador just arrived; lively in the morning, then listless at noon with diarrhea, vomiting, hiccups, no appetite

Lethargy, diarrhea, and vomiting in dogs can have many causes, including improper diet, parasite infection, and viral infection. From your description, symptoms appeared right after the dog arrived home, so stress cannot be ruled out and may be severe; a veterinary check is advisable. The vet will examine the dog thoroughly to determine the cause and provide appropriate treatment.