  We compute moments of non-diagonal correlators with two massive quarks.
Results are obtained where no restriction on the ratio of the masses is
assumed. Both analytical and numerical methods are applied in order to evaluate
the two-scale master integrals at three loops. We provide explicit results for
the latter which are useful for other calculations. As a by-product we obtain
results for the electroweak $\rho$ parameter up to three loops which can be
applied to a fourth generation of quarks with arbitrary masses.
