3.
已知函数\(f(x)=\sin (\omega x+\phi )(\omega > 0,0\leqslant \phi \leqslant \pi )\)是\(R\)上的偶函数,其图象过点\(M\left( \dfrac{3π}{4},0\right) \),且在区间\([0,\dfrac{\pi }{2}]\)上是单调函数.
\((\)Ⅰ\()\)求\(\varphi \)的值;
\((\)Ⅱ\()\)求\(\omega \)的值;
\((\)Ⅲ\()\)若\(f(\dfrac{\pi }{2}) < 0\),画出函数\(y=f(x)\)在区间\([-\dfrac{\pi }{8},\dfrac{5\pi }{8}]\)上的图象.