Leptons, which include particles like electrons, muons, and neutrinos, do not have a third component of isospin. Isospin is a concept in particle physics that was initially introduced to explain the similarities between protons and neutrons, which are both composed of quarks.
Isospin is a quantum number that characterizes the behavior of particles under the strong nuclear force, which is responsible for the interactions between quarks. It is analogous to spin, which is a quantum property of particles related to their intrinsic angular momentum.
Leptons, being elementary particles that do not participate in strong interactions, do not possess isospin. Isospin is primarily relevant to hadrons (particles composed of quarks) and their interactions. Leptons interact through other fundamental forces, such as the electromagnetic and weak forces.
So, while isospin is a useful concept for understanding the behavior of hadrons, it does not apply to leptons.