IEC 61804-3 is available from the IEC at https://webstore.iec.ch/publication/22636. FieldComm Group does not license this document directly.
However, this document is a superset of the HART and FF (E)DDL specifications. Developers can use those specifications to develop EDDs, but if you want the IEC version, then you would need to license it directly from IEC. For example, if you want to know more about PROFIBUS EDDs, then the IEC version would be appropriate.
FieldComm Group released an EDDL specification, in this edition, the IEC and FieldComm Group editions are closely aligned.
View the latest EDDL specifications by visiting https://www.fieldcommgroup.org/technologies/fdi/fdi-technology-detail#fdispecs