Electronic component mounting substrate, electronic device, and electronic module

An electronic component mounting substrate comprising:

• an insulating substrate comprising a recess that opens in a main surface of the insulating substrate, the recess for mounting an electronic component;
• a metal layer located on a bottom surface of the recess;
• an external electrode located on another main surface of the insulating substrate, the other main surface opposite to the main surface;
• a connection wiring located between the metal layer and the external electrode in a thickness direction of the insulating substrate;
• a plurality of first vias that connects the metal layer and the connection wiring and at least a portion of the plurality of first vias are located along a side wall of the recess in a perspective plan view;
• and a plurality of second vias that connects the connection wiring and the external electrode and that is located in a strip shape in the perspective plan view, wherein a notch is disposed in a side surface of the insulating substrate, and the connection wiring comprises a protrusion protruding to the side surface in which the notch is located in the perspective plan view.