It's kind of hard to say. Since it's the summer, I don't see much harm, since I would guess she is having plenty of downtime so there is not as much of a problem of being overscheduled, but at this age there is somewhat of a limit to how they will progress in most situations. It kind of depends on the gym and how fast they plan to move kids to more advanced levels, but the two classes per week will probably be working on the same skills at the same pace. The benefits will probably come from a strength, flexibility, and "timing" or motor standpoint. If your daughter is skating this summer then she will be getting an extra practice on the latter and some crossover aspects of the other two, and she could just as well do extra exercises at home.
Personally I think if she was skating I would not bother with an extra class...three times per week of kind of similar activity is a lot at that age and if she has enough unstructured time I think exposure to something of a little different nature might be better to think about overall, or even a more team oriented sport as opposed to individual. Something like music or art class, soccer or softball, etc. At that age I see some improvement even from kids who are doing a sport like soccer, simply because they are improving endurance, speed, focus, and coordination and also their ability to follow a practice and respond to directions. Even music or art will hone some of those less tangible skills while providing exposure to a greater variety.