  For a 3-qubit Heisenberg model in a uniform magnetic field, the pairwise
thermal entanglement of any two sites is identical due to the exchange symmetry
of sites. In this paper we consider the effect of a non-uniform magnetic field
on the Heisenberg model, modeling a magnetic impurity on one site. Since
pairwise entanglement is calculated by tracing out one of the three sites, the
entanglement clearly depends on which site the impurity is located. When the
impurity is located on the site which is traced out, that is, when it acts as
an external field of the pair, the entanglement can be enhanced to the maximal
value 1; while when the field acts on a site of the pair the corresponding
concurrence can only be increased from 1/3 to 2/3.
