Shaking is response of dog against pain. As your dog is shaking along with diarrhea and vomiting, his problem can be in gastrointestinal tract. One condition that causes abdominal pain along with diarrhea and vomiting is parasitic infection. Your dog need to be checked for intestinal parasites.
Another possibility of having all these three symptoms together is intestinal obstruction. Your dog might have ingested some thing that is has blocked his gastrointestinal tract. Another possibility of intestinal blockage is gastrointestinal tumors. So, take your dog to vet who may order for lab and radiological test.
Another possibility of having all these three symptoms together is intestinal obstruction. Your dog might have ingested some thing that is has blocked his gastrointestinal tract. Another possibility of intestinal blockage is gastrointestinal tumors. So, take your dog to vet who may order for lab and radiological test.