In the present work, we report on the combined experimental and theoretical studies of the 4f-5d spectra of Ce(3+), Pr(3+), Nd(3+), Eu(3+), Gd(3+), Tb(3+), Dy(3+), and Er(3+) ions in a newly synthesized K3YF6 matrix. The low temperature experimental 4f-5d excitation spectra have been analyzed and compared with the results of the energy-level and intensity calculations. For this theoretical analysis, the extended phenomenological crystal-field model for the 4f(N-1)5d configuration (i.e., the extended f-shell programs, developed by Prof. M. F. Reid) and exchange charge model (developed by Prof. B. Z. Malkin) have been used together to estimate the crystal field parameters and implement the spectral simulations. On the basis of the results of the performed theoretical analysis, we suggest the most probable positions occupied by optically active ions. Although the spectra of only eight lanthanide ions have been studied, the Hamiltonian parameters of the 4f(N-1)5d configuration have been evaluated for the whole lanthanide series and reported here for the first time, to give a complete and unified description of the spectroscopic properties of the trivalent rare earth ions in the chosen host. In addition to the studies of the 4f-5d transitions, various possible competitive excitation channels overlapping with 4f-5d ones have also been discussed, where a theoretical scheme giving rudiments to understand 4f-6s spectra are proposed for the first time. An excellent agreement between the calculated and measured excitation spectra shapes confirms validity of the performed analysis. The obtained parameters of the crystal field Hamiltonians for different ions and various electron configurations can be used in a straightforward way to generate the energy level positions and calculate the particular transition intensities for any rare earth ion in any particular spectral region. With the aid of the obtained parameters, the positions of the lowest energy levels of the 4f(N), 4f(N-1)5d ,and 4f(N-1)6s configurations of rare earth ions and 4f(N+1)(np)(5) configuration of rare earth ions and ligands (corresponding to the ligand-impurity ion charge transfer transitions) in the band gap of K3YF6 have all been estimated. The obtained Hamiltonian parameters and energy levels diagrams, which include the electronic structure of a host material, can be used as a starting point for analysis of spectroscopic properties of trivalent lanthanides in similar fluorides.