The CoSb3 skutterudite is a promising thermoelectric material with favorable electronic transport properties. Filling of the voids of CoSb3 skutterudites bears the possibility to reduce the thermal conductivity often without deteriorating much the good electronic transport properties. In this work, we present a systematic comparison of YxCo4Sb12 with Y = Ga, In, Tl by first principles. We perform this analysis by computing the lattice structure, the electronic structure and the electronic transport properties and by comparing to experimental values. Our results indicate that while In and Tl occupy the vacant 2a Wyckoff positions, Ga tends to substitute Sb atoms. We show that the gradual filling leads to a gradual n-type doping as well as a gradual change in the shape of the electronic band structure. Most of the change in the electronic structure is due to the increasing lattice parameter. However, for filling fractions that exceed x=0.04 the rigid band structure model becomes increasingly inaccurate. This indicates an additional interaction of the Sb4 rings with the filling elements. The latter is reflected in the atom projected band structure computations.