The source of the rumour is the Japanese website Macotakara. According to them, the iPhone 11 will contain three cameras that are grouped in a square camera hump. With another flash, that would come to 4 elements on the camera island. That reporting corresponds to renders that OnLeaks had previously made. Macotakara is said to have obtained the information from Chinese suppliers.
The render of Macotakara slightly contradicts the render of OnLeaks. On the render of OnLeaks, the cameras are not neatly aligned within the square, while this is true with the render of Macotakara. According to the Chinese website, Apple would opt for a similar design to the Huawei Mate 20.
According to the report, the triple camera would only be available on models that have a lot of storage memory. Now the form factor would not matter much, but whether or not it was equipped with a triple camera would depend on the amount of storage space. The 64 GB version of the iPhone 11 would, according to all expectations, get the same double camera as the current iPhone XS.