How does sampling and subsequent resampling affect aliased harmonics?
The alias behaves exactly the same, in every sense, as if it were an actual sinusoidal component at the alias frequency.

In your example, the 7 kHz harmonic sampled at 3 kHz is actually a 1 kHz sinusoid, and decimating it is exactly the same as decimating a 1 kHz signal. In this case, you end up with a DC value.