Translation:She goes to the gym three times a week.
When using the noun in its indefinite form, it becomes the "institution" rather than one specific unit. Which is more often than not correct use in Norwegian, especially with the "gå på" phrase which means "attending".
Jeg går på skole = "I go to school" ie I am a pupil and attend classes. Jeg går til skolen = "I walk to school" ie I walk to the building which is the school (I may or may not be a pupil/work there etc). The prepositions is often a way of differentiating the separate meanings. "Gå på" attending, "gå til" movement.