If you are getting your period once every three months this could be an indicator of something not quite right going on. If you are an older woman it could indicate that you are pre-menopausal. If you are of child bearing age it could mean that you are not ovulating properly. I would certainly check with your doctor.
I agree with the above you could be pre-menopause or abnormal ovulating patterns. However if you are a teenager and you've only been getting your period for a short time it's perfectly normal to skip a few months here an there, it's just you body trying to get used to a cycle. If this continues I suggest checking in with your doctor just to be safe!