Most postal or address systems are designed to work in 2D (e.g. ZIP codes or postal codes in the UK, or indeed latitude/longitude coordinates on which what3words is based). These always need additional information to specify height: e.g. Flat 6, 5th floor, 12 Lonsdale Road. With a 3 word address, we recommend a similar approach, e.g. Flat 6, 5th floor, table.lamp.chair
In terms of indoor addressing, what3words covers all locations in 2D, but the supporting technology (indoor positioning, indoor mapping, indoor navigation) is not yet widespread enough for there to be any great benefit of specifying a point with height included. Once you arrived at the entrance, your device would not be able to help you navigate to the indoor location (on whichever floor).