Ive never heard of that diagnosis before, maybe it would be worth talking to your doctor about your concerns?
Bipolar disorder can create a lot of depression and stress for its owner.
Do you find yourself stressed ? if so then i suppose it is more of a bipolar disorder than just depression. depression alone doesnt really make you stressed unless there are other factors. why noit try asking your doctor about medications?
Depression on it's own can be just as stressing as bipolar disorder, you can't base a diagnosis off of just stress.
I think that diagnosis is kind of controversial right now, but it does illustrate the way that bipolar illness may fall on a spectrum from straight up unipolar depression at point A to full blown manic depression at point B. Bipolar type 3 would maybe be depression with only "soft signs" of bipolar disorder. Take a look at this website to see what some of these are, and maybe you'll be able to identify more with those than the symptoms listed for distinct hypomanic or manic episodes: http://www.psycheducation.org/depression/02_diagnosis.html#soft
It sounds that your doctor thinks that because you experience a cyclical form of depression that there might be some sort of near-hypomania happening and that you would fall on the very far end of the bipolar spectrum. It does sound confusing, so ask the doctor that diagnosed you to explain exactly why they think you have bipolar depression instead of unipolar depression. Good luck!