Purpose of Having More ADC channels than ADC Pins on a Microcontroller
In the special case of the TMSC5515 you have 3 channels for 3 pins and 3 channels only for the pin GPAIN0.

Pin GPAIN0 allows higher voltages and provides a built-in voltage divider.

You may select the undivided voltage or the divided voltage by selecting one of the three channels. (See page 10 of the data sheet.)