All three companies say voice recordings are occasionally reviewed to improve speech recognition.
But the reaction to the Bloomberg article suggests many customers are unaware that humans may be listening.
Reviewers typically transcribed and annotated voice clips to help improve Amazon's speech recognition systems.
Amazon's voice recordings are associated with an account number, the customer's first name and the serial number of the Echo device used.
Some of the reviewers told Bloomberg that they shared amusing voice clips with one another in an internal chat room.
They also described hearing distressing clips such as a potential sexual assault. However, they were told by colleagues that it was not Amazon's job to intervene.
According to Apple's security policy, voice recordings lack personally identifiable information and are linked to a random ID number, which is reset every time Siri is switched off.
Any voice recordings kept after six months are stored without the random ID number.
Its human reviewers never receive personally identifiable information or the random ID.
Google said human reviewers could listen to audio clips from its Assistant, which is embedded in most Android phones and the Home speaker.
It said clips were not associated with personally identifiable information and the company also distorted the audio to disguise the customer's voice.