Italy is not the third oldest country. The concept of "oldest country" depends on definition (continuous sovereignty, statehood, civilization). Common rankings for oldest continuously sovereign states:
Other contenders: Denmark (unified 10th c.), Portugal (1143), France (843/987), UK (1707/1801), China (dynastic continuity but regime changes).
Italy unified as a nation-state only in 1861 (Kingdom of Italy); Republic since 1946. Its civilization (Etruscan, Roman) is ancient, but the state is relatively young.