Hi Gera. We would make several suggestions for you.
First for the bikini area, laser hair removal is the best option. You are never going to fix the dark spots until you get rid of the irritation associated with shaving and the best way to do this is to have laser hair removal. This may be the same issue with the underarm area, but difficult to tell from the photos.
On the back, we would suggest a prescription hydroquinone for the dark spots. If these are related to acne, you may need to address that issue with some antibiotics or topical products like Benzoyl Peroxide. The prescription hydroquinone (4% - 8%) should be able to take care of the spots, but if you are getting more with new acne, then it may not fix the overall problem.
Regarding the stretch marks on the chest. These are difficult to treat. We offer a laser called the Aramis for textured stretch marks but the lighter color is not possible to change as we cannot create more pigment for you.
Good luck.