Most postal or address systems only work in 2D (e.g. ZIP codes or postal codes in the UK). 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, index.home.raft
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.
However, if and when height becomes helpful to specify, we have various options for including this information with the 3 word address.