Sean Penn -- 3rd Divorce Attempt Is the Charm

Robin Wright Penn just filed for divorce from Sean Penn ... again ... making it the third time the couple has attempted to split thus far.

Robin filed the papers in Marin County, California on August 12, in which she explains that the couple has already decided to "share custody" of their 16-year-old son Hopper Jack.

According to the documents, obtained by People.com, Robin cites "irreconcilable differences" for the split -- even though they've already reconciled twice before.

The papers also explain, "the estranged couple have already agreed to division of all property."