Converting multiple geological TIN's to a single Solid UGRID

[huliogeordio4]

I have a number of geological layers as TINs. They are situated at their respective depths yet when I attempt to highlight them all and convert to solid or UGRID they create multiple grids (or solids) for each layer (TIN).

As I am new to the MODFLOW package, what is the simplest process for turning TINs (some do not cover the entire extent of the model) into a 3D block to model groundwater flow via particle tracking.

I have attempted fill between tins and horizons, however, nearly all solids appear as below,  mishapen and not extruding between the TINs as expected.