Tier 3 countries in advertising are the least desirable, and are mostly developing economies with citizens that have limited or no disposable income.
These are countries usually avoided by international advertisers. While the costs to advertise to these countries are low, conversion rates are abysmal.
You can copy the complete list of the tier 3 countries to use in any ad network and it is suggested that you exclude them from your advertising campaigns. The only exception being your business is specifically targeting this kind of customers or have a business at these countries.
If you want a full list of all country tiers, tier 1 countries, or tier 2 countries you can find them in the articles linked here.
Here is the full list you can copy and paste:
Albania
Algeria
Angola
Armenia
Azerbaijan
Bahrain
Bangladesh
Barbados
Belize
Benin
Botswana
Burkina Faso
Burundi
Cambodia
Cameroon
Cape Verde
Chad
Comoros
Congo
El Salvador
Ethiopia
Gabon
Georgia
Guatemala
Guinea
Haiti
Honduras
India
Iraq
Jamaica
Jordan
Kenya
Kuwait
Kyrgyzstan
Laos
Lebanon
Lesotho
Macedonia
Madagascar
Mali
Mauritania
Mauritius
Moldova
Mongolia
Mozambique
Namibia
Nicaragua
Niger
Nigeria
Pakistan
Senegal
Sri Lanka
Suriname
Swaziland
Tajikistan
Tanzania
Togo
Trinidad and Tobago
Tunisia
Turkmenistan
Uganda
Uzbekistan
Vietnam
Zambia