You can set the input format by selecting RAW8 from the drop down menu as shown above.
To set the output format as 24bit you have to choose 24-bit from the output video format drop down list. This will change CyU3PMipicsiDataFormat_t dataFormat as CY_U3P_MIPICSI_BUS_24 .
On other words, CyU3PMipicsiDataFormat_t dataFormat will be changed based on the Output video format that you will choose from the drop down menu.
In addition to this, you have to also change the GPIF II bus width to 24 bit.
Your answer confirms my third guess:
I configure CY_U3P_CSI_DF_RGB888 format for CX3, while the sensor output is sill RAW8
It means that sensor and CX3 data formats need not be aligned. If I want to use the full bus width I configure CX3 for RGB888 and make sensor output in any format I like (as long as host software can deal with it).