Component of the molecular motor that translocates viral genomic DNA in empty capsid during DNA packaging. Forms a tripartite terminase complex together with TRM1 and TRM2 in the host cytoplasm. Once the complex reaches the host nucleus, it interacts with the capsid portal vertex. This portal forms a ring in which genomic DNA is translocated into the capsid. TRM3 carries an RNase H-like nuclease activity that plays an important role for the cleavage of concatemeric viral DNA into unit length genomes. Interacts with the terminase subunits TRM1 and TRM2. Interacts with portal protein. Responsible for the nuclear localization of the two others subunits TRM1 and TRM2. Belongs to the herpesviridae TRM3 protein family.