looking for a gym or Aerobic class in Aluthgama or Bentota or kalutara

Whilst there may be a few gyms in the areas mentioned, I doubt very much if these will have aerobics classes and the like. Most gyms in these areas are probably pretty basic.

Your best bet would be to check with any of the numerous tourist hotels in the areas. I am sure they would have good gyms with a regular set of classes.

Best wishes.