The optical-microwave atomic-beam magnetic resonance technique has been used to measure four hyperfine transitions in the ${2}^{3}$P state $^{3}\mathrm{He}$. These were used to determine the three hyperfine interaction constants (in MHz) as follows: contact, C=-4283.${84}_{\mathrm{\ensuremath{-}}0.01}^{+0.02}$; nuclear-moment-electron-orbit, D=-28.06\ifmmode\pm\else\textpm\fi{}0.06; nuclear-moment-electron-momment, E=+7.10\ifmmode\pm\else\textpm\fi{}0.02. THe results, which are in excellent agreement with theory, give a clear indication of the effects of core polarization on the hyperfine structure. We conclude that to the accuracy indicated the structure of the 2 $^{3}$P state, including mass-dependet and quantum electrodynamic corrections to the fine structure, and relativistic and core-polarization corrections to the hyperfine structure, is well understood.