We can think of the "HgI"_3^"-" as being formed by the reaction
"Hg"^"2+" + 3"I"^"-" → "HgI"_3^"-"
The electron configuration of a neutral "Hg" atom is
"Hg"^0 = "[Xe] 6s"^2 "4f"^14 "5d"^10
For "Hg"^"2+", the electron configuration is
"Hg"^"2+" = "[Xe] 4f"^14 "5d"^10
The "Hg"^"2+" ion, however, has vacant "6s" and "6p" orbitals.
It can hybridize a "6s" and two of the "6p" orbitals to form three new vacant "sp"^3 hybrid orbitals.
These can each overlap with a filled "5p" orbital of an iodide ion.
Thus, the new ion is trigonal planar, and the "Hg-I" bonds are coordinate covalent bonds in which the iodide ions are contributing both electrons of the covalent bond.
