Parallel wires \(1\) and \(3\) are semi-infinite, so magnetic field at \(O\) due to them
\(\vec{B}_{1}=\vec{B}_{3}=-\frac{\mu_{0} I}{4 \pi R} \hat{k}\)
Magnetic field at \(O\) due to semi-circular \(arc\) in
\(YZ-\)plane is given by \(\vec{B}_{2}=-\frac{\mu_{0} I}{4 R} \hat{i}\)
Net magnetic field at point \(O\) is given by
\({\vec{B}=\vec{B}_{1}+\vec{B}_{2}+\vec{B}_{3}}\)
\({=-\frac{\mu_{0} I}{4 \pi R} \hat{k}-\frac{\mu_{0} I}{4 R} \hat{i}-\frac{\mu_{0} I}{4 \pi R} \hat{k}}\)
\({=-\frac{\mu_{0} I}{4 \pi R}(\pi \hat{i}+2 \hat{k})}\)