  We study the effect of electron-electron interaction on the surface
resistivity of three-dimensional (3D) topological insulators. In the absence of
umklapp scattering, the existence of the Fermi-liquid ($T^2$) term in
resistivity of a two-dimensional (2D) metal depends on the Fermi surface
geometry, in particular, on whether it is convex or concave. On doping, the
Fermi surface of 2D metallic surface states in 3D topological insulators of the
Bi$_2$Te$_3$ family changes its shape from convex to concave due to hexagonal
warping, while still being too small to allow for umklapp scattering. We show
that the $T^2$ term in the resistivity is present only in the concave regime
and demonstrate that the resistivity obeys a universal scaling form valid for
an arbitrary 2D Fermi surface near a convex/concave transition.
