Formation energies of C, Si, and Ge defects in beta-Ga2O3 are studied through hybrid functional calculations. The interstitial defects of these elements generally occur at higher energies than their substitutional counterparts, but are more stable at low Fermi energies in Ga-rich conditions, with their range of stability increasing from Ge and Si to C. In n-type and Ga-rich conditions, interstitials of Si and Ge show significantly higher formation energies than their substitutional form, but this difference is less pronounced for C. Charge transition levels of interstitial defects lie in the upper part of the band-gap, and account for several measured levels in unintentionally doped and Ge-doped samples of beta-Ga2O3.