Here’s a test: write a dinner party conversation for eight people, where everyone has different opinions, interests and agendas!
For many years of my writing life, I’d have baulked at that. Anything said can fly off in so many directions, it’s like juggling eight balls in the air. Now I relish that kind of challenge—because of the new dimensions that come into dialogue when additional speakers are introduced.
Say you have an intense two-way dialogue, a quarrel with recriminations between X and Y. Bring in Z—and perhaps X and Y have to conduct their quarrel with hidden barbs, under cover of polite appearances. Perhaps Z senses something anyway, and starts asking questions, or starts trying to smooth over the situation. Or, another possibility—perhaps X and Y turn to Z in self-justification, attempting to win her over to their version of events. Or perhaps X does that, while Y proudly refuses to appeal for sympathy…
The possibilities are only limited by the needs of your larger story. I’ve learned to enjoy the extra angles I can create by bringing in an extra person, or two, or three. There are times for straightforward dialogue, and there are times for variety and unpredictability.