First of all, make sure the geometry is suitable for mapped meshing (see the help file on this). This meshing will probably be suitable for the two plates - if they are of a "regular" geometric shape such as a square or rectangle - but not the fillet. Map mesh the two plates first and then free mesh the fillet. Remember to change the degenerate hex elements created at the interface to non-degenerate tets using the TCHG,ELEM1,ELEM2,ETYPE2 command.
Cheers,
-- drej --