  We examine the contributions from triple excitation cluster operators in the
relativistic coupled-cluster theory in atoms and ions. For this, we propose a
tensor representation of the triple cluster operator. Based on this
representaion and using diagrammatic analysis, we derive the linearized
coupled-cluster equations for single, double and triple excitation cluster
operators. The contributions from the triple cluster operators to the hyper
fine structure constants of single-valence systems Rb and Sr$^+$ are analysed
using the perturbed triples.
