Observations of Shubnikov-de Haas(SdH) and de Haas-van Alphen(dHvA) oscillations in organic metals (BEDT-TTF)2X, with X=KHg(SCN)4, θ-I3 and β″-AuBr2. are reported. In KHg(SCN)4 salt, in addition to the SdH oscillations with fundamental frequency of 670 T corresponding to about 16% of the first Brillouin zone(FBZ), we observed splitting of each SdH peak which we ascribed to “spin-splitting” We have also found that the ground state of this salt is not a simple metal but has some magnetic character. In θ-l3 salt we have succeeded in an observation of dHvA oscillations for the first time. We observed a “saw-tooth” dHvA oscillation characteristic to a highly two-dimensional and extraordinary clean electronic system. In addition to the fundamental frequency of 4170 T corresponding to 50.4 % of the FBZ and its higher harmonics, we observed an oscillation with lower frequency of 730 T corresponding to about 8.8 % of the FBZ. A new Fermi surface topology for θ-l3 salt is proposed based on the analysis of the dHvA effect. In β″-AuBr2, we observed complex dHvA oscillations, which can be explained in terms of the mixing of two fundamental frequencies of 47 and 268 T, suggesting the presence of very small pockets corresponding to 0.6 and 2.9 % of the FBZ.