What is a 3-ply yarn? A 3-ply is a yarn with three single yarns spun together. If the single was Z-spun, then you will S-ply, and if the single was S-spun, then you will Z-ply. The same goes for 4-ply or more-ply yarns—just add more plies! Why use multi-ply yarns?
The yarns stand up to abrasion and wear which is great for socks. The more rounded structure of the 3-plus-ply yarns make cable and other knit and purl stitch patterns pop.
Spinning more singles takes more time. Managing more than 3 or 4 plies can take practice and may require more than one lazy kate.
Judith MacKenzie suggests spinning all of the singles before you begin to ply. Survey singles for consistency. Spread inconsistent bits from less than perfect bobbins throughout the whole project while plying to even things out.