3.如圖,在Rt△ABC中,∠ABC=90°,AB=8,BC=6,動(dòng)點(diǎn)D從點(diǎn)C出發(fā),沿邊CA-AB向點(diǎn)B運(yùn)動(dòng),到點(diǎn)B時(shí)停止,若設(shè)點(diǎn)D運(yùn)動(dòng)的時(shí)間為t(t>0)秒,點(diǎn)D運(yùn)動(dòng)的速度為每秒4個(gè)單位長度.
(1)當(dāng)t=3時(shí),AD=
;
(2)用含t的代數(shù)式表示AD(AD>0)的長;
(3)當(dāng)點(diǎn)D在邊CA上運(yùn)動(dòng)時(shí),若△CBD是以BD或CD為底的等腰三角形,求t的值.