Try
u[x_,y_,z_]:= y;
v[x_,y_,z_]:= -d*y - UMAXJ*Tanh[k1*Sin[x] + k2*y + k3*z] + d2*z - Sin[x];
w[x_,y_,z_]:= d*y + UMAXJ*Tanh[k1*Sin[x] + k2*y + k3*z] - d2*z + Sin[x] +
mu1*(UMAXJ*Tanh[k1*Sin[x] + k2*y + k3*z] - d2*z);
Simplify[w[x,y,z]==0,u[x,y,z]==0&&v[x,y,z]==0]
which returns
mu1*Sin[x]==0