arcgis desktop - Combining Geological Grids and extruding between them?

I have a situation where I have 3 Geological grids, each showing a different horizon + the Seabed Grid.

I need to extrude between these grids in order to make the 3D polygon for the geological unit which is between these horizons. The problem occurs however when (due to Shallow gas or lower geological units protruding upwards) there are gaps in my grid; sometimes to the extent that one of the horizons only covers a small segment of the sea bed area at the surface. The solution therefore (I think) is to combine say, H3 + H2 for example, and H2 +1, and then: Extrude between [(H3+H2) (where H2 fills in the gaps of H3)] and [(H2 +H1)(Where H2 Fills in the Gaps of H1)].

The Situation becomes very complicated when some of H3 actually rises to the Seabed and the Seabed fills in some gaps in H3. A very complex series of combinations of grids is therefore needed involving probably H3, H2, H1 and the Seabed.

The only way I have found to do this is in Global Mapper, but version I have does not have the combine grids tool, unlike the expensive newest version. The software I Have access to include Arc GIS 10.2 and freeware such as Map window, QGIS, GRASS etc. The solution may involve raster calculator conditional function, however I have no experience in how to accomplish this task using this.