My nephew is only 3 years old and has the chicken pox. How many days will it take for them to go away?

Varies. They are considered contagious from the day before the rash started until the last one has scabbed & dried or a week later, whichever is longer. The healing depends somewhat on the kids scratching and any additional problems that can cause. most fade initially within 3 weeks but skin changes can last a year.
Perhaps a week. In most cases it takes upwards of a week before all the lesion's have crusted over. This is when one is generally no longer considered to be contagious. Sometimes it can take longer if there are a lot of lesions, shorter if there are only a few lesions.
7-14 days. 7-14 days, it becomes non-infectious when the vesicles scab over.