Multi-level Organization ACL Permissions
While it's more work to set up, I think you'd be very well served by taking advantage of multi-site capabilities of CiviCRM, especially in conjunction with the excellent Multi-site Permissioning extension.  That said - I'm very nervous recommending these tools to someone who hasn't set up CiviCRM before!  It takes a fair amount of web development know-how to get this all working.

This would get you two-tier permissioning for "free".  You need three tiers - local, national, international.  I'd probably give each local group their own site - using Domain Access for Drupal or Wordpress multisite.  Then I'd handle the national level via ACLs.  Hopefully someone might also give good advice on my related question here.

The alternative approach is to write a custom permission extension.  This has a performance benefit, and also leads to much easier management.  However, this requires a much higher level of skill!  I'd say the two shops with proven expertise here are Fuzion and JMA Consulting.  If you're a developer yourself, the Multisite Permissioning extension above is a good example to get you started.