That happens a lot in alliance raids. Either all three tanks want to be the big shot and they spin the boss around trying to take it from each other, or none of them want the job and the boss wanders around swatting everyone else. Which then leads to all three tanks calling each other out and the entire group getting annoyed.

Generally, if people start fighting over the boss, just let them have it. It's not worth the time arguing. Only a handful of alliance raid fights need multiple tanks anyway.