在平面直角坐标系\(xOy\)中,抛物线\(y=mx^{2}-2mx-3 (m\neq 0)\)与\(y\)轴交于点\(A\),其对称轴与\(x\)轴交于点\(B\)顶点为\(C\)点.
\((1)\)求点\(A\)和点\(B\)的坐标;
\((2)\)若\(∠ACB=45^{\circ}\),求此抛物线的表达式;
\((3)\)在\((2)\)的条件下,垂直于\(y\)轴的直线\(l\)与抛物线交于点\(P(x_{1},y_{1})\)和\(Q(x_{2},y_{2})\),与直线\(AB\)交于点\(N(x_{3},y_{3})\),若\(x_{3} < x_{1} < x_{2}\),结合函数的图象,直接写出\(x_{1}+x_{2}+x_{3}\)的取值范围为____________.