How do I measure a negative voltage with a ADC?
An inverting amplifier does not need a negative rail to invert the voltage.



Try to think of your power rails as what supply your output. If you look at the circuit, all op-amp pins are tied to a voltage of 0V or higher. When your range of -1 to -3 comes in, it will show up as the exact opposite of 1 to 3 on the output. This also gives you some advantages as a buffer, as the input impedance of your pin will not affect this circuit very much (so long as Rin||Rf is large).

I agree that a simple resistor divider does the job -- just letting you know that this also works.