  The study of the origin of heavy elements is one of the main goals of nuclear
astrophysics. In this paper, we present new observational data for the heavy
$r$-process elements gadolinium (Gd, Z=64), dysprosium (Dy, Z=66) and thorium
(Th, Z=90) in a sample of 276 Galactic disc stars ( --1.0$<$[Fe/H]$<$+0.3). The
stellar spectra have a high resolution of 42,000 and 75,000, and the
signal-to-noise ratio higher than 100. The LTE abundances of Gd, Dy and Th have
been determined by comparing the observed and synthetic spectra for three Gd
lines (149 stars), four Dy lines (152 stars) and the Th line at 4019.13 A (170
stars). For about 70% of the stars in our sample Gd and Dy are measured for the
first time, and Th for 95% of the stars. Typical errors vary from 0.07 to 0.16
dex. This paper provides the first extended set of Th observations in the Milky
Way disc. Together with europium (Eu, Z = 63) data from our previous studies,
we have compared these new observations with nucleosynthesis predictions and
Galactic Chemical Evolution simulations. We confirm that [Gd/Fe] and [Dy/Fe]
show the same behavior of Eu. We study with GCE simulations the evolution of
[Th/Fe] in comparison with [Eu/Fe], showing that unlike Eu either the Th
production is metallicity dependent in case of a unique source of the r-process
in the Galaxy, or the frequency of the Th-rich r-process source is decreasing
with the increasing of [Fe/H].
