Is scaling & polishing a good way to get teeth cleaned by the dentist every 3 mths?

Recommended. Even if you brush and floss regularly, getting a professional cleaning is a great way to keep your mouth healthy and odor-free.
Gum disease? We usually recommend scaling at 3 month intervals if someone has gum disease and we're trying to get it under control. Otherwise, scaling and polishing every 3 months is probably overkill. If you don't have gum disease, a cleaning every 6 months is a good interval to use.
That depends. if you gave periodontal disease, accumulate a lot of calculus (tarter) on your teeth, or have braces, then polishing and scaling every three months is a good way to help keep your mouth healthy. If you are in good oral health already and do not accumulate a lot of calculus, or have braces, then polishing and scaling every six months should be adequate.