While all three will remain stand-alone apps, at a much deeper level they will be linked so messages can travel between the different services.
Facebook told the BBC it was at the start of a "long process".
The plan was first reported in the New York Times and is believed to be a personal project of Facebook founder Mark Zuckerberg.
Once complete, the merger would mean that a Facebook user could communicate directly with someone who only has a WhatsApp account. This is currently impossible as the applications have no common core.
The work to merge the three elements has already begun, reported the NYT, and is expected to be completed by the end of 2019 or early next year.