Luminescence thermometry has garnered significant attention due to its rapid response and non-invasive nature. For practical multimodal thermometry that requires high sensitivity and anti-interference capability, single-system materials with high emission intensity are highly desirable. Herein, we report a three-mode optical thermometric material based on La3Mg2NbO9:Er3+, a double perovskite phosphor that exhibits intense green emissions centered at 527 and 547 nm under either 378 nm or 980 nm excitation. Co-doping with Yb3+ significantly enhances the up-conversion (UC) luminescence intensity of Er3+ by a factor of 3 and increases its down-conversion (DC) luminescence intensity by 5-fold. Furthermore, this phosphor demonstrates temperature-dependent sensitivity across UC luminescence, DC luminescence, and fluorescence lifetime modes. Thermometric performance evaluated via the fluorescence intensity ratio of the thermally coupled levels (2H11/2 and 4S3/2) of Er3+ reveals outstanding behavior in both DC and UC modes over a broad temperature range (298–573 K), achieving maximum relative sensitivities of 1.16 % K−1 and 1.19 % K−1, respectively. The fluorescence lifetime mode yields a maximum absolute sensitivity of 36.42 % μs K−1. With excellent temperature sensitivity across all three modes, La3Mg2NbO9:Er3+ exhibits considerable potential for applications in self-referenced optical thermometry.