Well, I came with an interesting proof. But I just want to verify it
$$\begin{array}{l} {\text { Applied at function } y=\sin x,} \\ {\text { We have, } \sin \left(\frac{A+B+C}{3}\right) \geq \frac{\sin A+\sin B+\sin C}{3}} \end{array}$$
From here we will get $$\sin A+\sin B+\sin C\leq \frac{3\sqrt3}{2}.$$
$$\begin{array}{l} {\text { Also by A.M. } \geq \text { G.M. in an acute angled triangle }} \\ {\frac{\sin A+\sin B+\sin C}{3} \geq \sqrt[3]{\sin A \sin B \sin C}} \\ {\Rightarrow \sin A+\sin B+\sin C \geq 3(\sqrt[3]{\sin A \sin B \sin C})} \\ {\Rightarrow \sin A+\sin B+\sin C \geq 3\left(\frac{\sqrt{3}}{2}\right)=\frac{3 \sqrt{3}}{2}>2} \end{array}$$
and from this I get $$\sin A+\sin B+\sin C\geq \frac{3\sqrt3}{2}.$$
Now the equation to be satisfied, only equality condition should hold.
So in an acute angled triangle $$\sin A+\sin B+\sin C=\frac{3\sqrt3}{2}.$$
Is there any fallacy in this convention.