Like others in the 3D FPGA market, TierLogic's focus is to site the configuration logic in a way that doesn't diminish the density of the logic array or affect the performance of the chip.
Another 3D FPGA company with a similar approach is NuPGA which says it buries the programming transistors in a foundation layer under the logic array.
Another self-styled 3D FPGA company is Tabula which uses the well-understood technique of dynamic re-configurability at sufficient speed to enable logic elements to be re-used in the same circuit. Can this be called 3D? Well Tabula rather cheekily takes the third dimension as being Time.
Other approaches to 3D FPGA use bonded wafers connected with internal vias - the kind of approach in which Tezzaron Semiconductor specialises.
The motivation for looking for new approaches to FPGA is the escalating cost - for diminishing benefits - of following Moore's Law
TierLogic is keeping schtumm until Wednesday but I did manage to elicit the gritted-teeth-sounding comment: "The Third Dimension Is Space not Time."