The spectral broadening of characteristics g-ray emission peaks from the reaction 12C(3He,pg)14N was measured in D(3He) plasmas of the JET tokamak with ion cyclotron resonance heating tuned to the fundamental harmonic of 3He. Peak shapes and intensities were analyzed on the basis of detailed Monte Carlo simulations in order to determine the 3He effective temperature and density. The results demonstrate the potential of high-resolution g-ray spectroscopy measurements as a probe of energetic ions in fusion plasmas.