If the eggs were fertile that she is currently sitting on then odds are they will hatch, 2-3 of 3. Don't put anymore eggs under her, being broody takes a lot out of a bird and you'd just be wasting eggs as when the first hatch she will abandon the nests and raise the new chicks.
A common practice if a person wants a hen to brood chicks is to collect the eggs, store fat end up on a counter, until you have the amount you want her to brood on. Make a few pencil marks on the eggs so you know those are the ones you placed under her. Other chickens will lay in a broody nest and broodies themselves will take eggs and place them into their clutch while brooding. With marked eggs you can easily tell which are newly placed and can take them out to eat if you check the nest every day and not allow them to start incubating. If I left for weekend or something would take out the unwanted extra eggs and break on ground for the rest of the flock to eat.
