We claim the ornamental design for a covering for a container of three tennis balls, as shown and described.
FIG. 1 is a lower side perspective view of a covering for a container of three tennis balls, showing our new design;
FIG. 2 is a side view thereof;
FIG. 3 is a front view thereof; and,
FIG. 4 is a bottom view thereof.
The ornamental design that is claimed is shown in solid lines in the drawings. The broken lines showing are for illustrative purposes only and form no part of the claimed design.