The dynamical behavior of purity and entanglement for excitions in three quantum dots placed in a cavity driven by a broadband squeezed vacuum is studied in the low exciton density regime. It is found that the excitions in the three quantum dots can evolve into a pure entangled state only when the dissipation of the cavity and excitions is negligible. When the injected field into the cavity is a squeezed vacuum, the excitions will be in a three-mode Gaussian entangled mixed state even in the long time limit. Analytical expressions are derived showing the dependence of the purity and entangleme...
