[ricm]

LED brightness drops off quickly once you get below the typical rated voltage, then it won't work when you get to the minimum. So your 3V supply for 1 LED rated for 3.1V isn't enough, which is why it looks dim. You need to give enough voltage to be somewhere between the typical rated voltage and the maximum. I would suggest taking power from the main battery pack (7.2V?) with a Y-lead, then have 2 of these LEDs in series (for left and right side) with a 20 ohm resistor. This will give each LED a voltage of around 3.3V - 3.4V which is between the typical and max ratings, should give you maximum brightness. For only a couple of LEDs this won't have any meaningful affect on battery life.

You can see the 50% power angle is quite narrow (15 degrees), so once you get to 15 degrees angle away from 'straight ahead', you lose half of the LED brightness - effectively making the 'light cone' 30 degrees. So its important to make sure the LED has good power supply to help give you the best chance of 'filling' the square reflector like you want.