将过去完成时的主动语态变为被动语态，需要遵循以下步骤：

确定主动语态的宾语：

在被动语态中，这个宾语将成为主语。

将主动谓语变为被动谓语：

使用“had been”加上动词的过去分词形式。

将主动语态的主语变为介词by的宾语：

如果需要指出动作的执行者，可以在被动语态中加上“by”短语。如果不需要指出执行者，可以省略“by”短语。

例句

主动语态：

I had given him three books.

变为被动语态：

Three books had been given to him by me.

主动语态：

They had finished the project before the deadline.

变为被动语态：

The project had been finished by them before the deadline.

主动语态：

John had eaten all the pizza before I arrived.

变为被动语态：

All the pizza had been eaten by John before I arrived.

主动语态：

The car had been repaired by the mechanic before it was sold.

变为被动语态：

The car had been repaired by the mechanic before it was sold.

通过这些步骤，你可以将过去完成时的主动语态有效地转换为被动语态。注意，当不强调动作的执行者时，“by”短语通常可以省略。