is there some reference that I'm missing here
Pavlov was a Russian scientist who studied conditioning and had a famous series of experiments where he trained dogs to salivate upon hearing the sound of a bell by ringing a bell right before feeding them for an extended period of time.
Basically he trained them to associate one thing (a Bell's ring) with another (food), eventually making them salivate every time they heard a bell sound.
In this chapter, we learned that by using the 3 cushion, she basically conditioned herself to have a reaction (horniness) to a stimulus (the number 3) just like the dogs salivating (reaction) on the ring of a bell (stimulus). We call this
Pavlovian conditioning.
TL;DR it's referencing a fairly well-known scientist's (Pavlov's) experiments on training dogs to react a certain way to a certain stimulus. Pavlov would roll in his grave because this is a stupid example of Pavlovian conditioning and not wholly accurate.