First principles calculations have been used to investigate the trends on the properties of isolated 3d transition metal impurities (from Sc to Cu) in diamond. Those impurities have small formation energies in the substitutional or double semivacancy sites and large energies in the interstitial site. Going from Sc to Cu, the 3d-related energy levels in the band gap move from the top of the band gap toward the valence band in all three sites. Trends in electronic properties and transition energies of the impurities, in the substitutional or interstitial sites, are well described by a simple microscopic model considering the electronic occupation of the 3d-related levels. On the other hand, for the impurities in the double semivacancy site, there is a weak interaction between the divacancy- and the 3d-related orbitals, resulting in vacancy- and 3d-related levels in the materials band gap.