  An appropriate framework for dealing with hadron structure and hadronic
physics in the few-GeV energy range is relativistic quantum mechanics. The
Bakamjian-Thomas construction provides a systematic procedure for implementing
interactions in a relativistic invariant way. It leads, however, to problems
with cluster separability. It has been known for some time, due to Sokolov's
pioneering work, that mass operators with correct cluster properties can be
obtained through a series of unitary transformations making use of so-called
packing operators. In the present contribution we sketch an explicit
construction of packing operators for three-particle systems consisting of
distinguishable, spinless particles.
