解：?$(1)$?將點(diǎn)?$A(3，$??$0)$?代入得?$-3^2+2×3+m=0$?
∴?$m=3$?
?$(2)$?∵二次函數(shù)?$y=-x^2+2x+3$?的對(duì)稱軸為直線?$x=1$?
∴另外一個(gè)交點(diǎn)為?$B(-1，$??$0)$?
?$(3)$?以?$AB$?為底，若?$S_{△ABD}=S_{△ABC}$?
則點(diǎn)?$C、$??$D$?到直線?$AB$?的距離相等
若設(shè)?$D(x，$??$y)，$?則?$y=±3$?
當(dāng)?$y=3$?時(shí)，?$-x^2+2x+3=3，$?解得?$x_{1}=0、$??$x_{2}=2$?
∴?$D_{1}(2，$??$3)$?
當(dāng)?$y=-3$?時(shí)，?$-x^2+2x+3=-3，$?解得?$x_{3}=1+\sqrt{7}、$??$x_{4}=1-\sqrt{7}$?
∴?$D_{2}(1+\sqrt{7}，$??$-3)、$??$D_{3}(1-\sqrt{7}，$??$-3)$?
綜上所述，點(diǎn)?$D$?的坐標(biāo)為?$D_{1}(2，$??$3)、$??$D_{2}(1+ \sqrt{7}，$??$-3)、$??$D_{3}(1- \sqrt{7}，$??$-3)$?
