. . . . . . . . longest river in . . . . . . . . world is . . . . . . . . Mississippi.

In the first blank, article "the" will be used because article "the" is used with superlatives.
In the second blank, article "the" will be used because 'world' is a proper noun.
Similarly, In the third blank, article "the" will be used because "Mississippi" is a proper noun.