3.
已知函数\(f(x)=2 \sqrt {3}\sin \dfrac {ωx}{2}\cos \dfrac {ωx}{2}-2\sin ^{2} \dfrac {ωx}{2}(ω > 0)\)的最小正周期为\(3π\).
\((I)\)求函数\(f(x)\)的单调递增区间;
\((\)Ⅱ\()\)在\(\triangle ABC\)中,\(a\),\(b\),\(c\)分别为角\(A\),\(B\),\(C\)所对的边,\(a < b < c\),\( \sqrt {3}a=2c\sin A\),并且\(f( \dfrac {3}{2}A+ \dfrac {π}{2})= \dfrac {11}{13}\),求\(\cos B\)的值.