The three Emperors and Empresses which ruled simultaneously were Charlemagne, the Emperor of the Holy Roman Empire (768-814), Irene, the Empress of the Eastern Roman Empire (752-803) and Tang Dezong, the Emperor of Tang dynasty China (779-805). Also Caliph Harun ar-Rashid was their contemporary.

Emperor Meiji, personal name Mutsuhito, is the odd one out. He was the founder of the modern Japan, and he ruled 1867-1912.

Charlemagne (Carolus Magnus, Charles the Great) was crowned as the Holy Roman Emperor in 800. His coronation ended what is known in Europe as the Dark Ages, and initiated the Carolingian Renaissance.

There was a marriage proposal between Charlemagne and Empress Irene. Should the matrimony have materialized, it would have re-united the Roman Empire for the first time after 395 AD.

Tang Dezong, personal name Li Gua (Li Kuo on the Wade-Giles Romanization) was the emperor of the Tang China. The Tang dynasty is known as one of the most illustrious dynasties of the Chinese history. Its capital was Chang'an.

More Info: en.wikipedia.org