  We present the analysis of the entire HARPS observations of three stars that
host planetary systems: HD1461, HD40307, and HD204313. The data set spans eight
years and contains more than 200 nightly averaged velocity measurements for
each star. This means that it is sensitive to both long-period and low-mass
planets and also to the effects induced by stellar activity cycles. We modelled
the data using Keplerian functions that correspond to planetary candidates and
included the short- and long-term effects of magnetic activity. A Bayesian
approach was taken both for the data modelling, which allowed us to include
information from activity proxies such as $\log{(R'_{\rm HK})}$ in the velocity
modelling, and for the model selection, which permitted determining the number
of significant signals in the system. The Bayesian model comparison overcomes
the limitations inherent to the traditional periodogram analysis. We report an
additional super-Earth planet in the HD1461 system. Four out of the six planets
previously reported for HD40307 are confirmed and characterised. We discuss the
remaining two proposed signals. In particular, we show that when the systematic
uncertainty associated with the techniques for estimating model probabilities
are taken into account, the current data are not conclusive concerning the
existence of the habitable-zone candidate HD40307 g. We also fully characterise
the Neptune-mass planet that orbits HD204313 in 34.9 days.
