Where in the world?
The flag of Yemen has three horizontal stripes of red, white, and black.

Yemen

اليَمَن

Population: 29,825,968

Region: Asia

Subregion: Western Asia

Capital: Sana'a

Top Level Domain: .ye

Currencies: Yemeni rial

Border countries

The flag of Oman has three horizontal stripes of white, red, and green with a red vertical stripe on the left containing the national emblem.

Oman

Population: 5,106,622

Region: Asia

Capital: Muscat
The flag of Saudi Arabia has a green field with white Arabic inscription and a sword.

Saudi Arabia

Population: 34,813,867

Region: Asia

Capital: Riyadh