Comma with and

The first sentence below I know is correct as it is out of the handbook. Are the second or third sentences wrong? (also should I have used a comma in this question?) The gymnast leapt several feet into the air, and then he did a triple twist and a somersault! The gymnast leapt several feet into the air, then did a triple twist and a somersault! ...or why use a comma at all? The gymnast leapt several feet into the air then he did a triple twist and a somersault!