I have just come across this blog post by Lauren at Lladybird and it's brilliant, as she explains that she only used a three thread overlock stitch to make up some Seamster Rose hip tights. I do not instruct this for my own pattern, but this is how I always use my overlocker, as I don't like to waste thread.
It's dead easy, as you just remove the thread from the outside needle. I don't even know the last time I had that threaded and there is half a broken needle on the outside position that I have never bothered to remove (lazy).
It definitely sews a finer seam, which is ideal for tights or close fitting garments and is (in my opinion) just as strong as a four thread overlock stitch, but you do have to be more careful about catching both layers as the stitch obviously has less reach into the seam.
I hope this helps!