The third meeting will happen sometime later where Miranda asks to meet you in an Apartment at the Presidium Commons, and she'll ask you for access to Alliance resources, which you'll want to grant by replying 'I trust you'. You'll do so at the Spectre Terminal: if you read Kai Leng's dossier, Shepard will directly tell Miranda about him, ensuring Miranda has her guard up. If you didn't complete Miranda's Loyalty Mission in Mass Effect 2, it's CRUCIAL that you read this at your Private Terminal, because you'll also get a message from Miranda asking to meet at the Citadel again. When you return to the Normandy's War Room, Anderson will forward you Kai Leng's dossier. The next stage will be after Priority: Citadel 2. Pick 'Nothing's changed' to continue the Romance with her. Eventually she'll ask if she's still a part of Shepard's plans for the future. The first time you can meet her is after Priority: Palaven, where she asks to meet you in Normandy Dock: Bay D24.
You'll meet with her three times throughout the game, and each meeting will turn out to be vital in ensuring she'll live after her investigation is complete. As you'll learn early into the game, a Romance with Miranda is a bit complicated because she's simultaneously on the run from Cerberus and investigating the disappearance of her sister Oriana.
