There is no "industry standard" - the specific symbology depends on the data vendor.
Might be ES3H, ES03H, or ESH3 and then there are the special characters some vendors use, e.g., @ES3H, /ESH3, /ES3H, us@ES03H, ES/H3, etc.
For example - Bridge, BMI, DTN, PCQuote, and others use ES3H or ES03H while Bloomberg, DBC, CQG and others use the ESH3 format.