3 storage locations?
I'm guessing you've got a 8gb phone (out of the box you get available 5.5gb).
Out of this 5.5gb it is partitioned into 1.5gb( / root ) and 4gb (internal storage sdcard0 )
/ Root storage is where apps you install is stored.
Internal storage is where appdata and gamedata is stored
You can also keep your files here.
Third location is sdcard1 or external sdcard 

You can learn more about it here