解：?$(1)$?∵二次函數(shù)的圖像經(jīng)過坐標(biāo)原點(diǎn)?$O(0，$??$0)$?
∴?$\ \mathrm {m^2}-1=0$?
解得?$m=±1$?
∴二次函數(shù)的表達(dá)式為?$y=x^2-2x$?或?$y=x^2+2x$?
?$(2)$?∵?$m=2$?
∴?$y=x^2-4x+3=(x-2)^2-1$?
∴拋物線的頂點(diǎn)為?$D(2，$??$-1)$?
當(dāng)?$x=0$?時，?$y=3$?
∴點(diǎn)?$C$?的坐標(biāo)為?$(0，$??$3) $?
?$(3) $?當(dāng)點(diǎn)?$P、$??$C、$??$D$?共線時，?$PC+PD$?最短，直線?$CD$?相應(yīng)的函數(shù)表達(dá)式為?$y=-2x+3$?
當(dāng)?$y=0$?時，?$x=\frac {3}{2}$?
∴點(diǎn)?$P $?的坐標(biāo)為?$(\frac {3}{2}，$??$0)$?
