  We study the entanglement properties of a molecular three-qubit system
described by the Heisenberg spin Hamiltonian with anisotropic exchange
interactions and including an external magnetic field. The system exhibits
first order quantum phase transitions by tuning two parameters, $x$ and $y$, of
the Hamiltonian to specific values. The three-qubit chain is open ended so that
there are two types of pairwise entanglement : nearest-neighbour (n.n.) and
next-nearest-neighbour (n.n.n.). We calculate the ground and thermal state
concurrences, quantifying pairwise entanglement, as a function of the
parameters $x$, $y$ and the temperature $T$. The entanglement threshold and gap
temperatures are also determined as a function of the anisotropy parameter $x$.
The results obtained are of relevance in understanding the entanglement
features of the recently engineered molecular $Cr_{7}Ni$-$Cu^{2+}$-$Cr_{7}Ni$
complex which serves as a three-qubit system at sufficiently low temperatures.
