There are many more then three tributaries and sub-tributaries to the Mississippi River but some of them include:
The Minnesota River.
The St. Croix River.
The Wisconsin River.
The Rock River.
The Iowa River.
The Skunk River.
The Des Moines River.
The Illinois River.
The Missouri River which includes the Platte as a sub-tributary.
The Ohio River which includes the Tennessee River as a sub-tributary.
The Arkansas River.
The Yazoo River
hope this helps.. good luck :)
