Three long, straight, parallel wires each carry a current of magnitude \(I\). The wires pass through the vertices of an equilateral triangle with side length \(a\), as shown in the cross-sectional view. Wire 1 carries current directed out of the page, while Wire 2 and Wire 3 carry currents directed into the page. What is the magnitude of the net magnetic field at the centroid (center) of the triangle in terms of \(I\), \(a\), and physical constants?