Turn an LED off when there is 3V input
A fairly simple method is this: -

Choose the MOSFET so that it turns on sufficiently when 3 volts is applied at the input. This usually means a VGS threshold value of about 1 to 1.5 volts.
The MOSFET could be replaced with a BJT and another resistor but you said &quot;simplest circuit&quot; and, using a BJT will require 3 components unless you are prepared to accept that the LED will be still turned-off at around 0.6 to 0.7 volts on the input.
But, perhaps the simplest way (no added components) is to break the LED connection to ground and wire the input as shown: -

This works because the Thevenin source voltage formed by the 12 volts, R1 and R2 is only 2.83 volts and hence, when 3 volts is applied to the LED cathode, it will slightly reverse bias the LED and turn it off.