Why does a triple composite occupy only one locus?

It's a single bound scene, so it hangs on one place like any other single image — but it carries three items.

* Fusing triples means one composite per locus — ~18 loci for a whole deck instead of 52. *

The palace doesn't care that the image encodes three items; to it, "Bond surfing on a cake" is just one picture to store at one locus. That's the compression in action: a 52-card deck becomes ~18 composite scenes needing ~18 loci, instead of 52. Fewer loci to build, fewer stations to walk, faster recall — all because three items ride in one image at one spot.

Go deeper:

• Method of loci (Wikipedia) — one image per locus — the rule that makes composites pay off.