This situation depends on several factors: species of swans, gender of third swan and individual attributes.

Some swans (individual attributes such as docile, calm) and some species of swans tolerate other swans better than others. Black swans are typically more defensive than other species, but if the ponds are very large, the smaller species may do better on the same pond.

Is the third swan a female? The male may be less defensive towards her, but then again there are no guarantees and he could try to mate with her and leave the first female out of the equation and be defensive to her.

Is the third bird an offspring from the pair. If so, then this might work out better than introducing a stranger.

There are no guarantees about whether this can work out because if one bird has a bad day and the odd bird out gets into the pairs' space, especially during breeding season, someone could get hurt or killed.

Also, the third bird may not want to be on the pond with the other swans and try to get back to its pond
thereby, setting it up to predators walking from pond to pond.

Unless, there is a major problem with the second pond and swan on it, we would suggest leaving well enough alone.

Should you try to introduce the third swan to the pairs' pond or ever try to get a mate for the third swan, you will need a pen for the introduction.

The pen will need to be fully enclosed top to bottom so predators cannot dig under or climb over to access the swan inside the pen. The pen should be 1/2 on water and 1/2 on land, approximately 12 feet width by 24 feet length with a feeder inside. The pen should be constructed so that it has some shade. The new swan is placed in the pen for approximately 2 weeks to get used to the new pond and the feeding system as well as allow all swans to be properly introduced. Observe for any signs of aggression towards the new bird or it towards the pair. If so, at the end of two weeks, there is still aggressive behavior, then this not going to work out. If no signs, then you can release, but have a boat nearby just in case something goes wrong and you have to make a rescue. Stay close by with boat for several days to a week and if no aggression, you should be okay. Again, no guarantees. You really have to be proactive
around nesting season because hormone levels can wreck havoc with behavior. The Regal Swan