Implementing ISO 27001 in different countries

For certification purposes, you can either have a single certificate covering the three sites or one certification for each site, but regardless of the adopted approach all sites will have to be audited to achieve certification.

Considering that all sites have to be audited, the best course of action would be to certificate each site at a time. This way you can better plan your expenses with implementation, and a problem with one certified company wouldn't affect the others.

These articles will provide you further explanation about ISO 27001 implementation:
- How to choose a certification body https://advisera.com/blog/2021/01/11/how-to-choose-an-iso-certification-body/
- ISO 27001 implementation checklist https://advisera.com/27001academy/knowledgebase/iso-27001-implementation-checklist/

These materials will also help you regarding ISO 27001 implementation:
- Book Secure & Simple: A Small-Business Guide to Implementing ISO 27001 On Your Own https://advisera.com/books/secure-and-simple-a-small-business-guide-to-implementing-iso-27001-on-your-own/
- Free online training ISO 27001 Foundations Course https://advisera.com/training/iso-27001-foundations-course/