Improved convergence theorems for bubble clusters. II. The three-dimensional case

Given a sequence $\{\mathcal{E}_{k}\}_k$ of almost-minimizing clusters in $\mathbb{R}^3$ that converges in $L^1$ to a limit cluster $\mathcal{E}$, we prove the existence of $C^{1,\alpha}$-diffeomorphisms $f_k$ between $\partial\mathcal{E}$ and $\partial\mathcal{E}_k$ that converge in $C^1$ to the identity. Each of these boundaries is divided into $C^{1,\alpha}$-surfaces of regular points, $C^{1,\alpha}$-curves of points of type $Y$ (where the boundary blows up to three half-spaces meeting along a line at 120 degree), and isolated points of type $T$ (where the boundary blows up to the two-dimensional cone over a one-dimensional regular tetrahedron). The diffeomorphisms $f_k$ are compatible with this decomposition, in the sense that they bring regular points into regular points and singular points of a kind into singular points of the same kind. They are almost-normal, meaning that at fixed distance from the set of singular points, each $f_k$ is a normal deformation of $\partial E$, and at fixed distance from the points of type $T$, $f_k$ is a normal deformation of the set of points of type $Y$. Finally, the tangential displacements are quantitatively controlled by the normal displacements. This improved convergence theorem is then used in the study of isoperimetric clusters in $\mathbb{R}^3$.