My daughter has three X sex chromosomes. Good to know they are not fatal!
Seriously, we were pretty concerned when we found out, but all the info I was able to find shows that it's pretty benign, other than they tend to grow tall (she's tall!) and tend to have a bit lower IQ than average. The same syndrome in men (XYY) I believe was once considered a "crime gene" because there were more men with this condition in prison than in the general population. My daughter has not shown any inclination for crime, and is graduating Cum Laude this May from college, so things are looking good.
On a funny note, when we found out, we told my dad, who had just gotten a computer and was connected to the Internet, so he decided to Google "XXX Sex chromosome." I told him before he hit "Enter" that he wasn't going to get the results he was looking for. In less then a second he got about 50,000 porn site hits!