The ornamental design for a three-dimensional sign, as shown and described.
FIG. 1 is a perspective view of a three-dimensional sign;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a back elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view of a three-dimensional sign according to the second embodiment, illustrated in an exemplary scenario.
The exemplary individual shown in broken lines in FIG. 8 forms no part of the claimed design. The specific placement of the individual in FIG. 8 is for illustrative purposes only.