What should you do if your 200A breaker panel is full and you need to add three forty amp two pole singal throw breakers?

First you need to determine if addition of the new breakers will exceed the 200A service under normal operating conditions. If so you will need to go to power company and increase service and put in a larger main panel. If you have the capacity for the extra current you can add a sub-panel and add the breakers there as well as the breakers you will remove from main panel to accommodate the sub-panel breaker. Another way depends on what breakers are in current panel. There are some breakers that can be duplexed in the same space as a single breaker.