Dy3+ single ions in CS3y2Br9 and Dy3+-Dy3+ dimers in Cs3Dy2Br9 were studied by optical spectroscopy and by inelastic neutron scattering (INS). In the diluted compound the energy level scheme for the eight crystal electric field (CEF) levels of the H-6(15/2) ground-state multiplet of Dy3+ in C3v symmetry is obtained and the CEF parameters are determined. In neat CS3DY2Br9 the effect of exchange interactions can directly be observed, both by optical and INS spectroscopy. The ground and first excited CEF levels are further split due to the exchange interaction between the two Dy3+ ions. These splittings can be accounted for by a Heisenberg Hamiltonian with an antiferromagnetic exchange parameter 2J = -0.05 cm-1.