3 days are good... It depends in which season u r coming.
Prefer tocomein rainy season (june to aug), youcan enjoy alot at tym. Don't come in summer... April to june.
You can visit all the good places in mumbai in 2 days and you can go for some good trek in lonavala aur near by khandala also.
Take 1-2 days extra, you will enjoy inhere.... Its cheap and awesome.
You can visit elephanta caves for which you will have to take ferry ride, spend evening on marine lines.
Do street shopping and enjoy street food.
visit Haji Ali dargah, mahalakshmi temple, siddhivinayak temple.
Spend time at bandra bandstand. Go to prithvi theater, prithvi cafe, Leopold cafe.
Enjoy your time in Mumbai!!!
You can start your journey in exploring beaches in Mumbai juhu beach in morning next to juhu beach eskon temple than malad Beach and last you can end your day with mira road khau gali.
second day you start you journey to shidhi vinayak temple, mahalaxmi temple and than go ahead to bandstand and enjoy the sea face and end your trip with ghatkopar khau gali.
third day marin drive , churchgate, gurgaon chopatty and shopping in churchgate and must eat kabab in near churchgate and fairy too.