You are trying to cope with two fundamental concepts (out of three) of the Dhammapada: anicca and anatta.
Anicca tells us nothing is stable, everything changes, which means a set is an artificial composition of items that are different every second. A set is a purely mental construction that is a perfect illusion about reality.
Anatta tells us nothing, no one has in any way a self, an essence, a soul, or whatever that could be said to be stable. The mind, citta, is a mental construct that changes every single second, widening, deepening, and the “essence” of it is impossible to define, and it is different from any person to any other person, and changing all the time in each person, following its own phylogeny in each person, and this phylogeny is not the same from one person to the next.
If you state these two concepts together in their impermanence and essenceless-ness then the third concept comes logically, dukkha, at any moment one item is “x” but at the same instant it is potentially dead because in the next iota-moment it will have changed, it will have died and been reborn as different. A collection of objects, no matter how artificial they may be, like 1, 2, 3, etc., is a totally non-definable entity, except in some purely artificial and abstract way that has no consequence on reality. How long will a set of three apples be a set of three apples? They will naturally decay with time, and Picasso might come along, as Jacques Prévert used to say, and eat one apple or more and only leaves the core on the plate.
That’s why the concept of infinity is a vain concept because we cannot define it, we cannot capture it, we can only say as we say about the universe: it is infinite because it is constantly expanding, but at any moment it is finite though a nanosecond later it is more infinite, though at this nanosecond later it is finite though expanding. This is only our mind that cannot cope with the reality of the universe. We are mentally compelled o define things as if they were eternal, but we know they are not and we know it is a vain quest. Imagine what three dogs might be in the mind of seven billion people in more than two hundred countries and cultures and more than three thousand languages. In some cases, the word, hence teh concept, of “dog might even not exist at all.
Dr. Jacques COULARDEAU