Three-person conversations, and any conversations involving more than two people, are notoriously difficult to write. Who are the main people in the conversation? Does everyone participate equally? How do I keep the third person in the reader's awareness when they're not saying anything, so they don't appear to come out of the blue when they do speak?
These are all good questions, and I've struggled through them just like everyone else, but I thought I'd share a few thoughts on the subject.
When I'm coming into any conversation, I want to make sure I start out knowing precisely the mood, motives, and voice/conversational style of each participant. This is critical even in a two-person conversation, because without this information, it's easy to have characters say things that the author wants them to say in order to get a story task accomplished - and if they're accomplishing author tasks, they won't come alive as characters and the whole conversation will fall flat.
Here are some questions you might want to answer about each person in the room where a conversation will take place:
How does this character feel (general mental state)?
How does this character judge each other person in the room?
What does this character want from each other person in the room?
What is this character's goal/purpose in the conversation?
What might this character stand to gain or lose depending on the success of the interaction?
And some voice and style questions:
Does this character feel comfortable speaking to others? In front of others?
Does this character enjoy or despise interruptions (or interrupt and hate to be interrupted?)?
Some of the answers to these questions won't just depend on the identity of the speaker, but also can change depending on circumstance. This is why it's so important to know how the speaker judges the other people in the room - a person asking a friend for something casual will speak differently from a person asking a colleague for an illicit favor in the presence of their boss.
Here are some examples of three-person situations.
1. Two people are talking to each other when another person arrives on the scene and tries to join in.
2. One person goes to ask a favor of another and brings a friend along for moral support.
3. Two people are talking when another arrives and decides that one of them has it all wrong and it's his job to explain the situation properly.
4. Two people are talking but a third is nearby and starts making relevant comments to indicate to the two that they're being overheard.
5. Two people are talking to each other and a third realizes that they aren't understanding each other at all, and tries to intervene to change the balance of their conversation (this happens in Cold Words between Rulii, Hada and Majesty).
In all of these situations, it's good to keep track of who the primary speakers are, and of the nature of the third's involvement. Each speaker will make conversational moves to align him or herself with the others, for particular reasons. A speaker will speak differently if she is intending her message to be heard by two people rather than one.
In my fantasy novel, for example, there's a situation in which my protagonist Dana's mother calls her on the telephone. This is ostensibly a two-person interaction, but because of the way the mother talks, halfway through the conversation Dana figures out that the mom is also making a show for Dana's sister, who is standing by invisibly on the other end of the phone. The cues to this are subtle - the mother makes comments about Dana's situation that are not quite fully appropriate to her, but have some ambiguity.
One thing to remember is that conversations aren't always entirely focused. A third person can interject something particularly relevant to their own situation, and the conversation can go in that direction for three or four turns, and then come back to where it was. Or someone can be distracted by thoughts of a third person in the middle of the interaction and become confused, as in the following example:
Evan turned to me. "Dana, what's this rumor I'm hearing about my dad?"
That took me by surprise. "Oh, well, he was looking for you, I think. But I told him if he wanted to see fireworks he should go down to the quad."
Above Evan's head, the first rally of fireworks soared up, blooming like flowers. I searched for Brian in the brightness but I couldn't see him. Thunder thudded into my ears.
"Yeah, well he's not here," Evan said.
I looked at him. "What?"
"My dad. He's not here."
This brings me to the issue of keeping track of a non-participant or low-level participant in a conversation. I have to admit that point of view and internalization do wonders in this regard. Even if a participant isn't speaking much, so long as the POV character cares about what he thinks, looks at him, or otherwise notices his presence, the reader won't lose track of him.
These are my thoughts at the moment, but I welcome questions and discussion to push the topic further.