Without further information it is very difficult for me to provide the best advice but I shall certainly try.
Please reply when you can and we can continue the conversation.
A loss of appetite can have several causes including dental disease, pancreatitis, a recent toxin ingestion, an infection, liver or kidney disease etc.
3 days is a long time and Bruno may have e.g. dehydration or low blood sugar
At this stage, a vet visit is best. They will examine him and may run some tests such as a blood test and abdominal scan. Treatment will depend on what is going on and may include e.g antibiotics, fluids, an anti nausea injection and assisted feeding.
At home, offer warm and tempting food such as mashed chicken or white fish.
Hopefully he is feeling better very soon.