The problem with a single LED replacement in the dome was that the LEDs, unlike the "stock" incandescent, are very directional. After putting in only one LED replacement, I have a very limited range of light, directly below the dome. After adding the extra 2, creating an array, it lit up the entire cabin with even light, much better.
One other thing to pay attention to is that the LED lights are polarized. This means that they only work in one direction. There is a "+" and "-" side to the bulb. If the bulb doesn't light up when you first install it, try reversing it in the socket. Also, if you you build an array as I have, test each LED to make sure you have aligned all the "+" sides together.
I hope this is helpful to some of you.