John Gill Commentary Joshua 21:7

"The children of Merari according to their families had out of the tribe of Reuben, and out of the tribe of Gad, and out of the tribe of Zebulun, twelve cities." — Joshua 21:7 (ASV)

The children of Merari by their families
The descendants of Merari, the third son of Levi:

[had] out of the tribe of Reuben, and out of the tribe of Gad ;
which were both on the other side Jordan:

and out of the tribe of Zebulun, twelve cities ;
in all forty eight, as the Lord commanded, (Numbers 35:7) .