  We calculate the effective action for disordered elastic manifolds in the
ground state (equilibrium) up to 3-loop order. This yields the
renormalization-group $\beta$-function to third order in $\epsilon=4-d$, in an
expansion in the dimension $d$ around the upper critical dimension $d=4$. The
calculations are performed using exact RG, and several other techniques, which
allow us to resolve consistently the problems associated with the cusp of the
renormalized disorder.
