We have explored the crystal structure and magnetic properties of two intermetallic solid solutions of the IrIn3 structure type. Although Fe0.95Co0.05Ga3 and Fe0.975Ni0.025Ga3 are isomorphous and possess exactly the same concentration of valence electrons, they display noticeable differences in their crystal structure associated with a different nature of the Fe–Co and Fe–Ni dumbbells. The former contains an odd number of electrons compared to an even number of the latter dumbbell. Responding to localized spins of the Fe–Co dumbbells, the 69Ga NQR relaxation rate in Fe0.95Co0.05Ga3 is governed by temperature independent spin-lattice relaxation typical for diluted paramagnetic centers at low temperatures. In Fe0.975Ni0.025Ga3 with zero magnetic moment of the dumbbells the 69Ga spin-lattice relaxation rate is strongly reduced and in-gap states in the energy gap show up as a broad maximum near 70 K.