The weight of shrimp varies substantially, so the number of shrimp in a 3-ounce quantity can vary. Shrimp are sold by count, indicating how many shrimp equal 1 pound in weight. For example, "jumbo" shrimp may be 16 count, meaning each weighs 1 ounce.
"Large" shrimp are usually 31/40 count, which means 31 to 40 shrimp make up 1 pound. This means there are around two or three shrimp per ounce in weight, which would mean six to nine shrimp in a 3-ounce serving. In addition, shrimp lose some weight when peeled and cooked, so nine or more cooked, large shrimp may be needed to make up a 3-ounce serving.