Hackers broke into The Washington Post’s servers and gained access to employee user names and passwords, marking at least the third intrusion over the past three years, company officials said Wednesday. The extent of the loss of company data was not immediately clear, although officials planned to ask all employees to change their user names and passwords on the assumption that many or all of them may have been compromised. Although company passwords are stored in encrypted form, hackers in some cases have shown the ability to decode such information. Post officials, who on Wednesday learned of the intrusion from Mandiant, a cybersecurity contractor that monitors the company’s networks, said the intrusion was of relatively short duration. “This is an ongoing investigation, but we believe it was a few days at most,” said Post spokeswoman Kris Coratti.