Mass selected cations of Ag-clusters are deposited, together with Ar seeded with CO$_2$ (or other electron scavengers such as O$_2$, N$_2$O, or CCl$_4$), on a cold substrate, while being flooded with low energy electrons. Optical absorption measurements reveal an efficient trapping of Ag cluster cations in the Ar matrix, provided CO$_2$ or another electron scavenger is present to ensure charge neutrality of the matrix. The spectrum of Ag$_3^+$ thus obtained is in excellent agreement with previous predictions from quantum chemical calculations.