When writing dialogue, writers tend to view this Rule of Three thing in an almost mystical way. It’s simple, though. First, don’t think of it as a rule as much as a helpful guideline or yardstick. If you should write four spoken lines without a break, the Dialogue Police are not going to show up at your desk. A guideline is just that.
Dialogue is often the first piece of a scene to come into focus in a draft. It doesn’t mean you can’t go back later and layer your scene with setting, action or emotion.
Writing Tip for Today: When you write dialogue, use the number 3 to help you do the following:

• Avoid creating the Talking Heads phenomenon no one likes to read. If the character speaks more than 3 lines without a break of some sort–either a new character speaking or some action, body language or interior thought or emotion, consider adding that break.
• If you write more than 3 exchanges of dialogue (the ping pong effect) consider breaking the conversation briefly with a bit of narrative. Notice I said a bit–no more than about 3 lines.
• If you must break dialogue to insert backstory or a flashback, keep your flashback to 3 lines, or 3 short paragraphs. The longer you veer away from the real time action the higher the risk that readers won’t remember it. This also technically called The Cold Mashed Potatoes Rule. It means that if in the real time scene your character is eating mashed potatoes and then recalls a painful event from childhood, the longer we stay in that childhood flashback, the colder those spuds become.

Next: How would your character say that?