Three flame retardants, tris(1-aziridinyl) phosphine oxide (APO), tris(2,3-dibromopropyl) phosphate (TDBPP), and bis(2,3-dibromopropyl) phosphate (BDBPP), are prohibited for use in textile products under the “Act on the Control of Household Products Containing Harmful Substances” in Japan. The currently used method for analyzing these substances by the Ministry of Health, Labor and Welfare in Japan (MHLWJ), is GC, which uses carcinogenic solvents and hazardous reagents during sample processing. Furthermore, due to the global shortage of helium gas used as carrier gas for GC, the development of an alternative method is necessary. In this study, we developed a method to simultaneously analyze the three flame retardants using LC–MS/MS. In addition, this method is an improvisation of ISO 17881-2, which involves extraction with acetone under acidic conditions and replacing it with acetonitrile. This prevents the acid decomposition of APO and the thermal decomposition of BDBPP. For accurate and sensitive quantification, deuterated compounds, APO-d₁₂, TDBPP-d₁₅, and BDBPP-d₁₀, were used as surrogate standards. The calibration curve displayed linearity within the 0.005–1.0 µg/mL range for APO and the 0.01–2.0 µg/mL range for TDBPP and BDBPP. The limits of detection for APO, TDBPP, and BDBPP were 0.19, 0.87, and 1.0 µg/g, respectively. These values were 2.2, 9.2, and 10 times more sensitive than the current detection limits of 0.4, 8, and 10 µg/g, respectively. The recovery of the three flame retardants in various textile products using the developed analytical method was 83.7–120.8%, which confirms their satisfactory analyses.