I have been looking for a 3 ply seed stitch newborn cardigan pattern to replicate one my late sister made for all newborn babies she knew.She made it so many times over the years that it was from memory and we have no hardcopy.I would like to restart the tradition in her memory and raise some funds for cancer charity so if anyone can help it would be much appreciated .This flickr photo is a very similar garment.
You can search on Ravelry.
Is it just a simple adaptation from DK pattern to 3 ply by knitting a tension square & changing needle size?
Yes, you need to adjust for gauge.
When using a thinner yarn (3-ply) and smaller needle than the pattern originally called for, you can often make a larger size of the original pattern (M) to get the intended size (S).
But, always check your gauge, as you may find that a pattern that calls for DK yarn, may use a thin DK and that your 3-ply (if it is thick or worked with larger needles) may get you correct gauge.
Look through the patterns at
http://freevintageknitting.com/baby-patterns/baby-sets-patterns
If she had the pattern at one time, it might have come from one of the old booklets or yarn wraps they reproduce (legally, I may add!)