She could give you just the chance with the Golden Mirror, no chance available after completing Flemeth's Grimoire. Speak with Morrigan and choose her over Leliana then speak with Leliana and choose her over Morrigan. Sharing the night with Zevran unlocks the 'Easy Lover' achievement. Unlike the love triangle with Morrigan and Leliana, when Morrigan expects the warden to deliver the news to the other party, the warden automatically breaks up with Zevran when choosing Morrigan. If they both hit high disposition at the same time, they will offer mutually exclusive romance options, and your choice will make the other one unavailable for the rest of the game. Edit It is in fact possible to maintain a romance with two or even three companions on the same playthrough without any jealousy conflicts.
Though you can have both active at high approval, this causes Zevran's dialogue to be bugged. You can also have Zevran if his approval is around 50 both Leliana and Alistair need to be hardened before attempting this.
