A new report from the Korean The Bell claims that Samsung's first foldable AMOLED phone will feature three 3.5" OLED displays, and will be folded inwards. Samsung expects to start producing the device towards the end of 2018, which means it will start shipping in early 2019.
The Bell also says that this device will not be a mass market smartphone - Samsung is expected to produce between 500,000 to 2 million devices, which means that its first foldable phone (like its first flexible phone, the 2013 Galaxy Round) will be a sort of limited technology-demonstration phone.
This report is quite confusing. It seems that when open, the phone will have a single display the size of a Galaxy Note display (so around 6.3-inch). The third display will only be used when the phone is closed, for notifications. According to earlier reports, the display when open will be larger, around 7.3". The Bell does say that the design is not final yet - and that Samsung is also looking into an out-foldable design.