I ran into the same situation as you 2 years ago before I set off for a long trip in my van. LED fixtures or commercially made products are very expensive and we still needed the energy savings because we had to rely exclusively on an 80W solar panel with 2 6V golf cart batteries.
I spent 2 weeks fiddling around, going to Radio Shack, buying circuit boards, etc., etc.
Here is my recommendation:
I built a total of 6 little LED lamps with wood fixtures (they are really nice) and my total power draw when all lights are on is 6 Watts. Basically almost inconsenquential. My total cost for complete lighting in my rig was 120$ including purchase of soldering iron.
1) You can find info, as I did, on the net on how to set up your circuits. It's very easy and you'll need some resistances to make up the difference between your 3 LED's in series at 10.8V total and your 12V supply.
2) Buy circuit boards, wire, and switches at Radio Shack or something similar.
3) Check out this website http://www.eled.com
They offer huge selection of LED's in all colors and sizes at very, very affordable prices. For example, a white LED at radio Shack was $4.00 and eled was selling them for $0.57 if you ordered more than 50.
4) Make sure you order LED's that are at least middle of the road in intensity. It's a bit more power draw but the lighting will be much nicer.
5) As a tip, I used small aluminum cake and pie pans as reflectors in my lamps.
Have fun and experiment and you'll end up with a much cheaper lighting system that draws practically zero power, will last for 10,000 hours and get lots of Ohhhs and Ahhhs from fellow schoolies.