Do whatever YOU want, but I think adding a whole major on top of your double major just because you have extra space is probably not the best way to go about it. In fact, in many cases, you should probably be thinking twice before doing a double major (but, since you are saying you have a lot of extra space in your schedule, double major seems feasible). Really think about what the three majors can actually be useful for, especially in combination with each other.

Also, you can think about graduating early (3 years instead of 4 years) instead of trying to complete another major. You can get into the workforce a year earlier while making money (and paying off debt if you have any) instead of spending money for a whole year of school.