The hybridization of I3 (Triiodide ion) is sp3d. The way we draw the structure of molecular compounds on paper is just a two-dimensional form. The understanding of chemical reactions and physical properties can be better understood with the visualization in a 3-dimensional platform.

Here are three possible arrangements of the electron pairs in I3-: Arrangement 1 I / \ I---I. In this arrangement, the central iodine atom is bonded to the other two iodine atoms. The three lone pairs of electrons are not shown in this diagram. Arrangement 2 I / \ I---I | | |

Here are three possible arrangements of the electron pairs in I3-: Linear Arrangement : All three iodine atoms and the two lone pairs of electrons are in the same plane. This results in a linear shape.

If you want to understand the I3- Lewis Structure, hybridization, polarity, and molecular shape then check this article on Triiodide molecule that explains it all.

This chemistry video tutorial explains how to draw the lewis structure of I3-. It also discusses the molecular geometry, bond angle, hybridization, and form...