  Sympathetic cooling of Th$^{3+}$ ions is demonstrated in an experiment where
$^{229}$Th and $^{230}$Th are extracted from uranium recoil ion sources and are
confined in a linear Paul trap together with laser-cooled $^{88}$Sr$^+$ ions.
Because of their similar charge-to-mass ratios the ions are closely coupled and
arrange themselves in two-species Coulomb crystals, containing up to a few tens
of Th$^{3+}$ ions. To show the suitability of the sympathetically cooled
Th$^{3+}$ ions for high-resolution laser spectroscopy, the absolute frequencies
and isotope shifts of 5F$_{5/2}$\,$\rightarrow$\,6D$_{5/2}$ and
5F$_{7/2}$\,$\rightarrow$\,6D$_{5/2}$ transitions of $^{230}$Th$^{3+}$ have
been measured. The system is developed for hyperfine spectroscopy of electronic
transitions of nuclear ground and isomeric states in $^{229}$Th$^{3+}$.
