You're using a select subset of many definitions for figure, number, and digit.
A digit is a single numerical valuenumeral (0-9), like the digit 5 in the number 5, or the digit 5 in the number 2507.
A number can have one or more digits, like 7 or 2507 or 2,507.38.
A figure can be a shape, a diagram, or a number; if used to describe a number, it can be composed of a single digit or of multiple digits.