Equal currents are flowing in three infinitely long wires along positive $x,y$ and $z$-direction. The magnetic field at a point $\left(0,0,-a\right)$ would be ($i$=current in each wire)

Magnetic field due to current in the wire along $z$-axis is zero. Magnetic field due to wire along $x$-axis is along $\hat{j}$ direction and magnetic field due to wire along $y$-axis is along $-\hat{i}$ direction. Both wires produce 

$B=\frac{{\mu }_0}{2\mathrm{\pi }}\frac{i}{a}$