Solution

When three identical ligands occupy one face of an octahedron, the isomer is said to be facial, or fac. In a fac isomer,any two identical ligands are adjacent to each other.
When three ligands and the metal are in one plane, the isomer is said to be meridional, or mer.It contains both trans and cis pair hence,it is also considered as a combination on trans and cis.
$$[Co(NH_{3})_{3}(NO_{2})_{3}]$$ show fac and mer isomers.