The ornamental design for a set of electrictoothbrushholders, as shown and described.
FIG. 1 is an exploded front view of a set of electric toothbrush holders;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a perspective view with the holders in use, the broken line showing of toothbrushes being for illustrative purposes only and forming no part of the claimed design.