Equalization system - 3 valves vs. 2 valves

[engmat2007]

Hi everyone.

In our standards we specified that gate valves (2" to 24") in steam service shall have an external 3 valve bonnet relief/equalization system. We got a suggesting to use an external 2 bonnet relief/equalization system instead of 3. What do you think?, which configuration is the best?, which configuration is more commonly use in the industry?. What is the code to design this type of gate valves?

Thanks!