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 said it was a 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 or Instagram 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.
The services will continue to operate as stand-alone apps, but their underlying technical infrastructure will be unified, said four people involved in the effort. That will bring together three of the world’s largest messaging networks, which between them have more than 2.6 billion users, allowing people to communicate across the platforms for the first time.