The UNION keyword will return unique records on the result list. When specifying ALL (UNION ALL) will keep duplicates on the result set, which the OP don’t want.
SELECT city FROM tableA
UNION
SELECT city FROM tableB
UNION
SELECT city FROM tableC
- SQLFiddle Demo
RESULT
╔════════╗
║ CITY ║
╠════════╣
║ Krakow ║
║ Paris ║
║ Rome ║
║ London ║
║ Oslo ║
╚════════╝