$O$ is centroid and using the $\triangle O A D$ distance $O D=\frac{a}{2 \sqrt{3}}$
By all the three sides $AB , BC$ and $CA$, direction of magnetic field produced will be same and inward to the plane of paper
So $B_{\text {total }}=3\left[\frac{\mu_0 i}{4 \pi\left(\frac{a}{2 \sqrt{3}}\right)}\left(\sin 60^{\circ}+\sin 60^{\circ}\right)\right]=\frac{9 \mu_0 i}{2 \pi a}$

