fall over、fall down都是倒下、倒塌的意思.
例：The toddler tried to walk but kept falling down.那小孩学着走路但老是跌跤.
She fell over and broke her leg.她跌倒後摔折了腿.
fall off是向下倾斜或者数量或质量下降的意思.
例：Beyond the hill,the land falls off sharply towards the river.山那边地面呈陡坡向河边倾斜.
Attendance at my lectures has fallen off considerably.听我讲课的人已大大减少了.谢谢你