Understanding many-to-many ternary relationship sets

I am new to entity-relationship diagrams. I'm a bit confused when it comes to interpreting ternary relationship sets such as this: Does this signify that every relationship instance of Party will have one fighter, one mage, and one healer? If we were simply dealing with binary relationship sets without key or total constraints, every instance of the relationship would be linked to an entity from each entity set. But in the case above, isn't it possible to not have one instance from a particular entity set? E.g. if the Party instance only had one fighter and one healer (and no mage)?