I haven't had my period but I have taken 3 pregnancy test all say negative.. What could be causing the missed cycle

Well either you're not pregnant or your HcG levels aren't high enough yet.

A ton of things could be causing a missed cycle. Namely stress. Most women have a missed cycle here and there, they're called anovulatory cycles and aren't really anything to worry about unless you miss more than one. In which case you should see your doctor.

Also, are you having any other symptoms abdominal pain/fullness? Change in sleep patterns? Hair changes?

... also 2 other possibilities, you aren't testing correctly, or you are a rare person who gets pregnant but tests negative.

Most women skip a period here and there or are late, but you don't say how much unprotected sex you are having, or if protected, what kind.

Are you at least 14 days late, If not, then you are testing too early, are you testing with first urine of the morning?
And some women do not show positive till 3 month also.

Or of course there are dozens of other reasons women miss a period, it could be one of those