Arduino M0 multi channel ADC scanning

I've been trying to configure the ADC on the M0 for multi-channel ADC scanning. My requirement is three channels, single ended. So I know that from the datasheet, I need to configure the INPUTCTRL reg for number of channels and type of channel (POSMUX):

ADC->INPUTCTRL.reg = ADC_INPUTCTRL_GAIN_1X | // ADC gain factor x1
                         ADC_INPUTCTRL_INPUTSCAN(0x0)| // Number of channels included in scan
                         ADC_INPUTCTRL_MUXPOS_PIN0 | // Positive MUX input selection
                         ADC_INPUTCTRL_MUXPOS_PIN2 | 
                         ADC_INPUTCTRL_MUXPOS_PIN3 |

My question is how do I control the process of next channel that's going to be scanned. The only interrupt I can set is RESRDY which indicates the ADC result is ready but does not indicate which ADC channel the result is from. I did'nt understand this process from the datasheet, so can somebody share their insight on implementing this function?