SYDNEY, Dec 9 (Reuters) - Virgin Australia expects the profit outlook for the Sydney-Melbourne-Brisbane triangle will be challenging for a long time as Regional Express Holdings Ltd (Rex) enters the market, Virgin’s chief executive said on Wednesday. Rex plans to launch Sydney-Melbourne flights in March in a challenge to Qantas Airways Ltd and Virgin and has begun selling seats for as little as A$79 ($58.70) each way to gain customers by undercutting its rivals. “It is a challenging outlook for the profit pool on the triangle for a long period of time because Virgin Australia has no intent of backing off that market share either,” Virgin CEO Jayne Hrdlicka said at a CAPA Centre for Aviation event.