Translation:I work out in a gym two or three times a week.
I've talked with a Dutch-speaking Belgian friend and he didn't agree to use them, referring to a gym when you want to practice fitness. Then he suggested fitnesscentrum. He also recommended me to use the verb fitness maken or fitnessen in place of sporten.
What do you think about his opinion?
That is probably a regional preference. "Fitnesscentrum" and "fitnessen" would definitely be understood in all of the Netherlands, but they are not the preferred terms (in the area around Amsterdam, at least). "Fitness maken" sounds very Flemish to me and might raise some eyebrows here.