Horizon3 has raised $250 million in Series E funding at a valuation exceeding $2 billion, giving the cybersecurity company substantial new capital to expand its autonomous penetration-testing platform and move further into automated defensive remediation. The round was co-led by existing investors NightDragon and New Enterprise Associates (NEA). Horizon3 says its valuation has more than tripled from the $650 million valuation attached to its Series D just over a year ago. The financing arrives…