What should be switching voltage rating of relay in a 380VAC 3 phase system?
The right way to do this is with a pair of mechanically and electrically interlocked three pole contactors (With auxiliary contacts fitted to provide the electrical interlocking) such that it is physically impossible for one set of contacts to close until the others have fully opened. 

Depending on where you are located this thing may need to be certified by someone, in which case buying a "Three phase Automatic transfer switch" is the way to bet.

If building your own a "Three pole mechanically interlocked motor reversing contactor" (It is actually a pair of contactors with an interlock module) with an NC aux contact on each half can usually be easily modified to do what you need.  

I would start by speaking to your generator vendor as they will probably be able to either supply a packaged solution it or tell you who to speak to, and can also provide useful things like automatic generator start controllers and generator starting battery float charge systems (And oil heaters if required). 

This is totally standard data centre stuff. You may wish to take advice as well as to how often you need to run the generator on load to get best reliability (A machine that has been left standing for two years will very likely NOT start when you need it to).

Answer put together from a comment thread that ran long.