Third country operators are those operators outside of the EU who wish to operate within Single European Sky airspace.

UAS operators that have their principal place of business, are established, or reside in a third country, shall comply with all the rules documented in the IR
The competent authority for the third-country UAS operator shall be the competent authority of the first Member State where the UAS operator intends to operate
Competency from 3rd country is valid if the Commission, after consultation of EASA, has ensured that the requirements on the basis of which such certificates have been issued provide the same level of safety as this Regulation

Small Print: These posts are a summary only, include my personal interpretation and are not intended to be a replacement for reading the actual rules.