$\mathcal{N}=3$ conformal superspace in four dimensions

We develop a superspace formulation for ${\cal N}=3$ conformal supergravity in four spacetime dimensions as a gauge theory of the superconformal group $\mathsf{SU}(2,2|3)$. Upon imposing certain covariant constraints, the algebra of conformally covariant derivatives $\nabla_A = (\nabla_a,\nabla_\alpha^i,\bar{\nabla}_i^{\dot \alpha})$ is shown to be determined in terms of a single primary chiral spinor superfield, the super-Weyl spinor $W_\alpha$ of dimension $+1/2$ and its conjugate. Associated with $W_\alpha$ is its primary descendant $B^i{}_j$ of dimension $+2$, the super-Bach tensor, which determines the equation of motion for conformal supergravity. As an application of this construction, we present two different but equivalent action principles for ${\cal N}=3$ conformal supergravity. We describe the model for linearised $\mathcal{N}=3$ conformal supergravity in an arbitrary conformally flat background and demonstrate that it possesses $\mathsf{U}(1)$ duality invariance. Additionally, upon degauging certain local symmetries, our superspace geometry is shown to reduce to the $\mathsf{U}(3)$ superspace constructed by Howe more than four decades ago. Further degauging proves to lead to a new superspace formalism, called $\mathsf{SU}(3)$ superspace, which can also be used to describe ${\mathcal N}=3$ conformal supergravity. Our conformal superspace setting opens up the possibility to formulate the dynamics of the off-shell ${\mathcal N}=3$ super Yang-Mills theory coupled to conformal supergravity.