There are two typical types of dressings used with sunomono a vinegar based dish; one is nihaizu and the other is sanbaizu. Nihaizu is a combination of vinegar, soy sauce and dashi. Sanbaizu is nihaizu to which a small amount of sugar or mirin is added.
The definition of sanbaizu is equal parts, vinegar, soy sauce and mirin, however in actuality, it is typically made with a 3:1:2 ratio of rice vinegar, soy sauce and sugar. There are many recipes that call for different combinations.