I am having an odd problem with this too. I carried over an ME1 relationship with Liara and a ME2 relationship with Miranda. Near the beginning of the game Liara asked me to choose so I chose her. But when I first met Miranda she asked we whether we were still together. I saved the game here. If I say yes to Miranda does that end my Liara relationship? I know that I will need to choose ultimately, but I want the benefits of the Miranda relationship because apparently (very mild spoiler) having a relationship with her changes the end of a certain mission (not the last mission). So is it possible to juggle the two relationships for a time before an ultimate choice at the end? Sort of like ME2.