I romanced Liara in ME1 and the Lair of the Shadow Broker during Mass Effect 2. What would happen if I romanced someone else after the Lair of the Shadow Broker in ME2 in terms of ME3?
The person that you were last engaged in a romance with is taken on into Mass Effect 3, however I believe you only have one chance to romance someone per game (as the final romance "lock in" is just before/after going through the Omega relay).
Because you don't romance Liara (you just carry on the romance) during Lair of the Shadow Broker you can still only romance one "new" person from ME2 (by new, I mean start a new romance).
Hope that helps.