  We construct 3d, N=2 supersymmetric gauge theories by considering a
one-parameter `R-flow' of 4d, N=2 theories, where the central charges vary
while preserving their phase order. Each BPS state in 4d leads to a BPS
particle in 3d, and thus each chamber of the 4d theory leads to a distinct 3d
theory. Pairs of 4d chambers related by wall-crossing, R-flow to mirror pairs
of 3d theories. In particular, the 2-3 wall-crossing for the A_2
Argyres-Douglas theory leads to 3d mirror symmetry for N_f=1 SQED and the XYZ
model. Although our formalism applies to arbitrary N=2 models, we focus on the
case where the parent 4d theory consists of pairs of M5-branes wrapping a
Riemann surface, and develop a general framework for describing 3d N=2 theories
engineered by wrapping pairs of M5-branes on three-manifolds. Each 4d chamber,
which corresponds to a dual 3d description, maps to a particular tetrahedral
decomposition of the UV 3d geometry. In the IR the physics is captured by a
single recombined M5-brane which is a branched double cover of the original UV
three-manifold. The braiding of branch loci and the geometry of branch sheets
play a key role in encoding the physics.
