Does impetigo cause scarring?

Generally not. Impetigo is a superficial skin infection, when healed you are left with a mild redness for a couple of weeks. Unless the infection is deep, such as carbuncles or inflamed cysts, scar generally does not result. Impetigo, by definition, is superficial, thus does not generally leave scars.
It can. Usually it will not, but if it invoves deeper tissues, as cellulitis or abcess, it may well. Of course, people who form keloids (large scars) are much more susceptible.
Not usually. Impetigo usually does not cause scarring. Some people may get some slight darkening or lightening of the skin after the infection but that usually goes away after a few weeks.