  Starting with the generic Ginzburg-Landau expansion from a microscopic
$N$-band model, we focus on the case of a 3-band model which was suggested to
be relevant to describe some iron-based superconductors. This can lead to the
so-called $s+is$ superconducting state that breaks time-reversal symmetry due
to the competition between different pairing channels. Of particular interest
in that context, is the case of an interband dominated pairing with repulsion
between different bands. For that case we consider in detail the relevant
reduced two-component Ginzburg-Landau theory. We provide detailed analysis of
the ground state, length scales and topological properties of that model.
