这三个都是被动短语,可翻译成“据说”,知识时态有区别而已.我各加一个主语造个简单的句子吧,这样有助于你的理解.
He is said to come.据说他会来
He is said to be coming,据说他正在来的路上.
He is said to have come,据说他来过了.