Using a SS and FF Constraint to set duration

I am trying to create a task that is of uncertian duration. The variable lenght task starts (SS 0d lag) with the begining of a second task and ends (FF 0d lag) when a third task is completed. The task's length is dependant on the start and finsh of the other two tasks.

When I enter the task I get a task that is one day long and starts and finishes the day before the FF constraint.

Is it possible to make a task fo flexible lenght? If so, how do I do it? 

Thanks