The ornamental design for a three sided toothbrush, as shown and described.
FIG. 1 is a top, front perspective view of a three sided toothbrush showing my new design;
FIG. 2 is a top, rear perspective view thereof;
FIG. 3 is a side elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a top, front perspective view of the toothbrush further illustrating the environment in which it is used;
FIG. 8 is a side elevation view of the toothbrush as shown in FIG. 7; and,
FIG. 9 is a bottom plan view of the toothbrush as shown in FIG. 7.
The broken lines showing a handle in all of the views and elements on the head in FIGS. 7-9 illustrate environmental structure that forms no part of the claimed design.