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