  This paper combines several new constructions in mathematics and physics.
Mathematically, we study framed flat PGL(K,C)-connections on a large class of
3-manifolds M with boundary. We define a space L_K(M) of framed flat
connections on the boundary of M that extend to M. Our goal is to understand an
open part of L_K(M) as a Lagrangian in the symplectic space of framed flat
connections on the boundary, and as a K_2-Lagrangian, meaning that the
K_2-avatar of the symplectic form restricts to zero. We construct an open part
of L_K(M) from data assigned to a hypersimplicial K-decomposition of an ideal
triangulation of M, generalizing Thurston's gluing equations in 3d hyperbolic
geometry, and combining them with the cluster coordinates for framed flat
PGL(K)-connections on surfaces. Using a canonical map from the complex of
configurations of decorated flags to the Bloch complex, we prove that any
generic component of L_K(M) is K_2-isotropic if the boundary satisfies some
topological constraints (Theorem 4.2). In some cases this implies that L_K(M)
is K_2-Lagrangian. For general M, we extend a classic result of Neumann-Zagier
on symplectic properties of PGL(2) gluing equations to reduce the
K_2-Lagrangian property to a combinatorial claim.
  Physically, we use the symplectic properties of K-decompositions to construct
3d N=2 superconformal field theories T_K[M] corresponding (conjecturally) to
the compactification of K M5-branes on M. This extends known constructions for
K=2. Just as for K=2, the theories T_K[M] are described as IR fixed points of
abelian Chern-Simons-matter theories. Changes of triangulation (2-3 moves) lead
to abelian mirror symmetries that are all generated by the elementary duality
between N_f=1 SQED and the XYZ model. In the large K limit, we find evidence
that the degrees of freedom of T_K[M] grow cubically in K.
