Mesopotamia has the Euphrates and Tigris Rivers.
Nile is the main river of Egypt.
The Hebrews moved to more than just three places, but the three main places were: Cana'an (כנען) Egypt (מצרים) Babylon (בבל)
Ebro, Tajo, Guadalquivir, Guadiana, and the Duero are Spain's main rivers.
Cavally, Sassandra, Bandama and Comoé are the four main rivers.
Mekong,Salween,Irrawaddy.
Thames Avon Medway
Three of Egypt's major trading partners include the USA, Italy and the UK.
Don't know if I am answering your question correctly. But to my understanding we call them "sub-rivers" or "tributaries" which will merge into the main rivers.
The stadium was named due to its location at the confluence of the Allegheny River and Monongahela River, which forms the Ohio River.
The Pyramid Age is another name for Egypt's Old Kingdom:Because Egypt had three kingdom ages.
Egypt Italy and Spain