A Superior Court judge on Thursday dealt a devastating blow to Partners HealthCare's plans to expand its dominance across Eastern Massachusetts, rejecting a controversial deal that would have allowed Partners to acquire three community hospitals and add hundreds of doctors to its network. After months of deliberations, Suffolk Superior Court Judge Janet L. Sanders sided with opponents of the acquisitions, who argued that allowing Partners to expand more would give the health system, already the state's largest and most expensive, the market power to further increase costs. It's highly unusual for judges to reject consent judgments, and Sanders' decision came as a surprise to many in the health care industry who had expected the settlement to be approved.