The regiospecific characteristics of n-3 polyunsaturated fatty acids (PUFAs) in triacylglycerol (TAG) significantly affect the physicochemical and physiological properties of marine fish oils. In this study, the TAG molecular species composition and positional distributions of fatty acids were investigated in three marine fish species rich in omega-3 PUFAs (anchovy, tuna, and salmon). The regiospecific distribution of the fatty acids was measured with the allylmagnesium bromide (AMB) degradation method. The TAG compositions were analyzed with HPLC and the TAG molecular species were identified with APCI/MS. DHA was preferentially distributed at the sn-2 position of TAG, whereas EPA was evenly distributed along the glycerol backbone. The combinations of FAs, DDO, EOP, EPS, DSS, OOS, and PPS were the predominant TAG molecular species, and OOP, DOS, and DPoPo were the characteristic TAG molecules in the anchovy, salmon, and tuna, respectively. These data can be used to distinguish other marine fish species. The TAG composition categorized by TCN and ECN showed well-structured distributions, with double or triple peaks. These findings should greatly extend the use of marine fish oils in food production and may significantly affect the future development of the fish oil industry.