I2C is a bus. The expander helps with the loading aspects on the bus once 4 or more sensors are added.
Take a look at the iOS and Android source here: WICED Sense Table of Contents for examples which include the processing of the sensor data within an application.
I'm not aware of any headers on WICED Sense kit which allow direct access to I2C (just the test points) as this kit was not really designed to be a development tool.
TP3 connects to GPIO P2/Physical Pin 31 per the datasheet here: CYW20737S Bluetooth Low Energy SiP Module Technical Reference
Thank you for answer me the questions! Btw, our team is learning how the sensor kit works than we will focus on the development kit.