How to calculate thickness of MS plates required to fabricate a water tank of 3000mmX3500MMx5000MM, open from top side and how much pressure shall be on its side and bottom plates?

 

Since it is an open vessel, you don't need to worry about a pressure rating.

This is not really something you can design in an email.

You can use 18 gauge sheet metal with the right frame.

Pressure is related to how high of a column of water you have at a
particular point.