From cyclotron frequency ratios of HD^{+}/^{3}He^{+}, HD^{+}/T^{+}, and T^{+}/^{3}He^{+} we measure the mass difference between atoms of T and ^{3}He to be 1.995 940 8 (23)×10^{-5} u, corresponding to a Q value for tritium β decay of 18 592.071(22) eV. This enables an improved check on systematics of β decay experiments that set limits on neutrino mass. Using the HD^{+} mass calculated from the atomic masses of the proton and deuteron as given by Rau et al. [Nature 585, 43 (2020)NATUAS0028-083610.1038/s41586-020-2628-7], we also obtain improved atomic masses for the triton and helion (considered to be fundamental constants), namely, 3.015 500 716 066 (39) and 3.014 932 246 957 (38) u.