Please read the descriptions of the traditional caches first. Each one will have part of the coordinates for this bonus cache (northling in one, eastling in the other).
This cache is somewhere along the two paths. I shan't tell you which one to go for first so you may feel like cursing me if you go for the wrong one first! :) This is a much smaller cache than the other two but, like the others, it is hidden just off the path.
Congratulations to powerscape for FTF for the whole series, only about an hour after the caches were placed (thanks to Red Duster's efficiency in publishing them).