Sentence Correction

Here I have three statements:

1) She is thought to be secretly wealthy.

2) She thinks of them as heroes.

3) They are thought of by her as heroes.

#1 and #2 are correct, while #3 is incorrect. Why is #3 incorrect?

There is no explanation provided; my guess is that "by her" is a prepositional phrase instead of noun, which makes it an incorrectly constructed idiom. To fix, I would think to move "by her" outside. However, can not use that construction since I don't have a noun. So I use construction from statement #1: "They are thought to be heroes by her."

Am I correct in my reasoning here?