Layering works best when each necklace has a little room of its own. Different lengths help individual pieces stay visible instead of collecting around the same point.
You do not need three or four necklaces to make the idea work. Two can be enough: one sitting slightly higher and another falling lower.
From there, decide whether you want the combination to feel balanced or deliberately mixed. A simple chain next to a more noticeable pendant can create contrast without making the overall look feel busy.
And if one necklace already does the job on its own, leave it there. More is an option, not a requirement.