  The soft anomalous dimension governs the infrared singularities of scattering
amplitudes to all orders in perturbative quantum field theory, and is a crucial
ingredient in both formal and phenomenological applications of non-abelian
gauge theories. It has recently been computed at three-loop order for massless
partons by explicit evaluation of all relevant Feynman diagrams. In this paper,
we show how the same result can be obtained, up to an overall numerical factor,
using a bootstrap procedure. We first give a geometrical argument for the fact
that the result can be expressed in terms of single-valued harmonic
polylogarithms. We then use symmetry considerations as well as known properties
of scattering amplitudes in collinear and high-energy (Regge) limits to
constrain an ansatz of basis functions. This is a highly non-trivial
cross-check of the result, and our methods pave the way for greatly simplified
higher-order calculations.
