She is said to have been born in the 3rd century BC. (fine)
People say she was born in the 3rd century BC. (fine)
Both of the above sentences mean the pretty much (there are fine nuances) the same thing.
People say she has been born in the 3rd century BC. (not fine)
She has been (verb) is an ongoing process, or it's a process that occurred in the past but has ongoing ramifications continuing into the present.
She has been dead for twenty centuries.
She died and is dead and will still be dead tomorrow. This is ongoing, but lots of people are still dead. The reason this is used (as opposed to she died twenty centuries ago is predominantly because there is some significance today:
She has been dead for twenty centuries, yet even today she is remembered as the face that launched a thousand ships.
She has been traveling for twenty months.
It's ongoing. Or, there's something more, today.
She has been traveling for twenty months, but it seems she only left a months ago.
People say she has been born in the 3rd century BC. (not fine)
You can't be born in an ongoing manner in the past. Birth may take a long time, but usually less than twenty-three centuries.
People say she has been born again. (fine)
People say she has been reincarnated. (fine)
What is ongoing is a spiritual experience, or that she was born into a new body from a previous life; or it may have a consequence today:
People say she has been born again because of her wonderful life of merciful works.
Please see this excellent post on tenses in English.