Connect extension wires by soldering.. what i did in a similar case is cutting the 3 conductor wires to different lenghts (about half an inch difference) to keep the individual joints between the conductors and extensions away from each other. First isolate the solder joints individually, then tie the 3 wires together over the lenght where the joints are to make cable handling a bit easier. I used muticore wire for the extensions, the same thickness as the original conductor cable. Keep the solder joints as thin as possible so it's not too much hassle to get them through the holes in the wood.
There's cool stuff for sale in electronics- or possibly scale model-shops for isolating the joints, it's like a tube that you slide over the solder joint and then if you heat it up gently with a torch or paintstripper-hairdryer kind of device it shrinks to a tight fit around the solder joint. You even can buy the stuff in a larger diameter to tie the 3 conductors together that way. Unfortunately i don't know the correct name in english for that stuff..
H.