One Round Debates are better
Three Round Debates are better
No, the 1 round is a quicker way for 2 people to put all their ideas on 1 paragraph and move quickly to the voting session it would be much easier and quicker.
I agree with jreslen
One rounders are too short.
3-rounders give people more space for aguing and linking.
For one round debates, you pretty much only hear each other's opinions and that's not exactly how a heated debate would go, but in a three round debate, you have time to counter argue and actually try to change each other's opinions.
Rebutting and defensing are needed.