In my experience, the coverstitch stitches what looks like twin needle straight stitch on the right side and then a ‘back and forth’ stitch on the wrong side. It is therefore un-necessary to first stitch a 3 thread wide on the edge and then fold it up and stitch a coverstitch.
This double stiching probably created the stretching you encountered.
I have simply folded up the hem or cuff, and then stitched the coverstitch–lining up my right serger needle so it would stitch through both layers of fabric, and the left serger needle so it stitches just to the left of the turned up raw edge which one can feel through both layers of fabric as it is fed into the machine. that way the ‘back and forth’ of the cover stich neatly covers the raw edge on the wrongside. If you look at ready wear this is how hems and cuffs on knit wear is done. eliminates a step.
If one does not have a coverstitch machine (or serger with that capability) then one would indeed start with a serger three thread wide on the raw edge or a sewing machine zig zag, turn up the cuff or hem, and then stitch on the regular sewing machine a twin needle stitch.
