I would say the biggest con with three is you only have two hands, so walking, carrying them etc requires a little more thought.
Having more than two definitely won't make your existing dogs love you any less. Domestic dogs are not actually pack animals, they consider themselves part of your family no matter how many there are.
The advantage with the possible new dog being an adult is you can take her out with your two first and see how they respond to each other. If all goes well you could take her on trial to see how she fits in.