Hi Bhawna , 

     In comparison to all three, I would recommend MGM Medical College, Navi Mumbai as it is established in 1989, whereas Sharda is fairly new medical college established in 2009. It is better to join a medical college that is well established, since they have been MCI recgonized and been through all the recognition hassles. MGM is known for it's good management and excellent faculty. Sharda University is located in NCR region, it stays mostly 40-45 degrees there. If you are okay with hot temperatures then go for it, but if you have discomfort or you are coming from a cold region then definitely pick MGM Mumbai. Mumbai is much more friendly and safer then Greater Noida where Sharda is located. 

   Besides my opinion, at the end it matters what you prefer, if you are ok with the big city feel, rains, humidity, women safety then definitely go for MGM Mumbai. Or else Sharda University is also good, it's just not that old , fairly new medical college so please make sure it has received recognition from authorities. Besides that you won't be able to explore much in Greater Noida as it is not as big of a city as Mumbai. 

Hope this helps!

All the best! :)