> I am using LPC845 BRK and need to configure ADC2 and ADC3 to measure voltage levels from pin27 and pin8.
I don't have/use a LPC845 myself.
But, reading the datasheet, I think you got that wrong. This MCU has only one ADC, which can have up to 12 separate channels.
To use these channels, you would need to initialize the respective GPIO pins similiar channel 0 (which I think the mentioned example provides).
> ADC_DoSelfCalibration(ADC2_BASE, frequency));
And a calibration is done for the whole ADC unit, not individual channels.