Want a 3v led to operate when 12v is applied
It's hard to tell from your question, but apparently you have a 6 V power feed and a 12 V power feed to a circuit, and you only want a LED to come one when the 12 V feed is active, not the 6 V feed.  And there is a diode from the 6 V feed to 12 V feed?

I guess you're asking the question because if you put a resistor in series with the LED sized to get the brightness you want when 12 V is applied, you still get too much brightness when 6 V is applied.

There are two relatively easy ways to deal with this:

Put another diode, preferably a Schottky, in series with the 12 V supply and run the LED off the voltage before the diode.

Use a transistor as a threshold detector set to around 9 V so that it is on at 12 V but off at 6 V, and run the LED thru the transistor.