  We analyze the linear thermoelectric transport properties of devices with
three quantum dots in a star configuration. A central quantum dot is
tunnel-coupled to source and drain electrodes and to two additional quantum
dots. For a wide range of parameters, in the absence of an external magnetic
field, the system is a singular Fermi liquid with a non-analytic behavior of
the electric transport properties at low energies. The singular behavior is
associated with the development of a ferromagnetic or an underscreened Kondo
effect, depending on the parameter regime. A magnetic field drives the system
into a regular Fermi liquid regime and leads to a large peak ($\sim k_B/|e|$)
in the spin thermopower as a function of the temperature, and to a $\sim 100\%$
spin polarized current for a wide range of parameters due to interference
effects. We find a qualitatively equivalent behavior for systems with a larger
number of side coupled quantum dots, with the maximum value of the spin
thermopower decreasing as the number of side-coupled quantum dots increases.
