Here are tips for introductions:
http://www.theguineapigforum.co.uk/showthread.php?t=38562
Please be aware that adding a third piggy does not always work out; trios are the most difficult combination to get right without ending up with an outsider problem of some kind in the longer term. A well bonded couple does not always like a newcomer.
Let the newbie live next to your girls for a few days so she can settle in, get to know her future mates through the bars and has as many extraneous stress factors taken out of the equation when you come to the intro proper. You can stage the intro over several days, but you always need to give it several hours in one go or the piggies won't be able to work out their relationship. Interrupt as little as possible.
Generally, the acceptance phase takes about 5-20 minutes from first interaction which should be fairly quick - a plate of food or hay helps to bring them together. Then it moves straight into the dominance phase if acceptance has happened. Any instant hostilities (teeth chattering or worse - have a look at the boar behaviour thread for the list of aggressive behaviours) or a refusal to interact is a bad sign. Things can blow up during the dominance phase if the wrong signals are given, so you need to keep an eye on things if things are not going quite as smoothly. Some of my girls have enjoyed just toeing this side of the line for a whole day... mallethead
Best of luck!