Make My own Output Variable in Energyplus?

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As alternative of what Aly suggested, , always through EMS:

1- create 3 EMS:Sensors to take in your 3 variable (same as Aly suggested);

2- write a simple EMS:Program to sum the 3 Sensors: ZoneTot = Zone1 + Zone2 + Zone3 (same as Aly suggested);

3- call the program through a EMS:CallingManager : my suggesiton Before Zone Time Step reporting

4- create an EMS:OutputVariable that calls the EMS variable sum of the three Sensors (ZoneTot),

5- than create a standard Output:Variable (that could sounds like ZoneTotFromEMS), with "*" as Key Value and the EMS variable name (ZoneTot) as Variable Name

Please see steps 4- and 5- in the screenshot as exapmle for the sum of Chillers Cooling Energy