when he came back with plent of money,his father had been dead three days before对吗?

不对， 表示已经死了3天应该用 had been dead for three days
而如果是3天前死了，可以用died three days ago.

最好的说法是 When he came back with plenty of money, his father had already been dead for three days.

应该是不对的，这是个状语从句，when he came back with plenty of money, 是一个过去时态的时间状语作从句，除了单词“plenty”拼写错误之外没什么问题，而主语要表达的意思是”他的爸爸在他回来的三天前已经过世“，跟从句的时态一致的话就应该用过去完成时态，had + 动词的过去分词，这里就有问题了, been dead 是一个状态，后面就应该接一个时间段”for three days“， 而非时间点”three days before，另外die 是一个瞬时的动,不能用在这里，这里正确的表达应该是
“When he came back with plenty of money, his father had been dead for three days.”

不对，been 应去掉，要么就改成had been dead for three days , have been + 动词ed指动作一直持续的时间，不能和before 连用
呵呵希望能帮到你

不对,后面句子是完成时,所以不能用 before,三天前
应该这样说
when he came back with plent of money, his father had been dead for three days