None of the Scandinavian countries have a name with only three letters.
England
Belfast is the only city in Ireland with seven letters in its name. Many, many towns and villages have seven letters in their name, but no other city does.
There are many cities in the country of Spain. There are also many cities in Spain whose name only have 5 letters. Ainsa, Alcoy, Altea, Caspe, and Elche are just a few of the cities who have 5 letters in their name.
BarbadosBotswanaBulgariaCambodiaCameroonColombiaDjiboutiDominicaEthiopiaHondurasKiribatiMalaysiaMaldivesMongoliaPakistanParaguayPortugalScotlandSlovakiaSloveniaSri LankaSurinameTanzaniaThailandZimbabwe
Sweden Norway
Yes. Its the name of the only principal city within, and the name of the country.
minsk
The largest country in the world is Russia and has more than four letters, but the largest continent in the world is Asia with four letters.
Yes. It's a country that consists of only one city. Both the country and the city have the same name - Luxemburg.
Only 1. The city you are looking for is Johannesburg.
Liechtenstein is the only one I know. ;)