There are approximately nine types of fish with 3-letter names. These names are: ayu, cod, dab, gar, eel, koi, ide, oto, and ray.
The ayu is also called the "sweetfish", because its flesh has a sweet taste to it. It's native to the waters around and within Japan, China, Taiwan, Hong Kong, and the Korean Peninsula. Cod is the common name for the entire genus of fish known as Gadus. Of these nine fish, it's the most popular as a food for humans, and it's also used to make the commonly used nutritional supplement cod liver oil.
The dab is an edible flatfish native to the North Sea. The name "eel" refers to an entire order of elongated fish, which have poorly developed fins and resemble snakes. The gar is a freshwater fish of North America with an elongated body and long snout.
The word "koi" refers to the ornamental varieties of common carp, which are often used for decorative purposes in ponds and gardens. The ide is a freshwater fish from northern Europe and Asia, while "oto" is a nickname for a genus of catfishes native to South America, which have an armored body and feature a suckermouth. Ray is a common name for marine fishes of the order Rajiformes or Batoidei, such as stingrays, skates, and manta rays.
There may be a few other less common 3-letter names for fish, which are either regional, newer, or more obscure nicknames.
In a recent study by the nonprofit group Oceana, it was found that 1 in 5 of all fish sold in U.S. restaurants and markets are wrongly labeled. For example, out of 120 samples labeled red snapper and bought for the study, 28 different species of fish were found.