The Diakoptics (Segmentation) procedure and its application to wideband dispersive absorbing boundary conditions have been implemented in the 3-D TLM method with symmetrical condensed nodes. We show how a microwave structure can be partitioned into substructures which are solved independently and later reassembled, all in the time domain. Frequency dispersive non-TEM boundaries are represented in the time domain by their characteristic impulse response or Johns Matrix. Parasitic reflections due to finite space and time discretization steps have been reduced to less than one percent by exponentially tapering the impulse response of frequency dispersive boundaries, allowing wideband S-parameter extraction from a single impulsive TLM simulation. The S-parameters of some 3-D waveguide discontinuities computed with this technique compare well with the available data.