To use the MEAN Function 

MEAN(Tag1,Tag2,Tag3)

The mean is found by adding up all of the given data and  dividing by the number of data entries. (Giving you the average)

Tag1 + Tag2 + Tag3 / 3

MEAN(Demo-influxdb/DEMO_01TI031.PV,Demo-influxdb/DEMO_01TI032.PV,Demo-influxdb/DEMO_01TI021.PV)