Question

_____ is the successor of the largest 3 digit number Fill in the blanks to make the statements true.

Answer 1

1000  is the successor of the largest 3 digit number.

Answer 2

Successor should be the "next consecutive number" for example 3 is the successor of 2, 4 is the successor of the 3 etc.  

Largest three-digit "number is 999", the successor of "largest 3-digit number" is 999 + 1 = 1000.

The successor is generally the next that is the next possible occurrence to the preceding number. If the "given number" is 9, then the successor for the given number will be 10. This is how it goes.