Chemical pregancies are incredibly common, the statistics vary greatly, just these two websites give different statistics from 25% to 70% I haven't looked at the research statistics properly myself, but most of the fertility/recurrent miscarriage consultants I have seen tend to quote 60% for early (less than 6 weeks) miscarriages.
I know that each of the recurrent miscarriage specialists (I have seen many) that I have talked to, tend to disregard any miscarriage before 6 weeks as being of any significance at all. They are only interested in miscarriages after 8 weeks. This is essentially because they are so common.
They are common because it is normal, I think people test early and know earlier than they used to in years gone by, so people are now aware they occur, whereas previously people would not have known they occur so often.
I have had 12 miscarriages that I know of (like others I could have had other chemical pregnancies that I do not know about!). Three of these were chemical prenancies in that I did not get scanned before I miscarried. Five more were confirmed by scan but miscarried before 6 weeks. Only 4 of them were of any interest to the recurrent miscarriage consultants as they were after 6 weeks with heartbeat present. Most miscarriages occur before the heartbeat starts, which is why they say that after a heartbeat is seen the miscarriage risk reduces to less than 5%.
The common miscarriage before 6 weeks is generally assumed to be due to genetic issues and not of any significance as it is "normal"! If you are concerned then you could get blood tests to check, thyroid, blood clotting etc. If you are getting pregnant then the day 3 and day 21 tests are not so helpful (low progesterone is under dispute as to if this is a cause or result of miscarriage), progesterone supplimentation is being tested currently as it if it helps at all. My experience of progesterone is that it delays the miscarriage but does not prevent it.
I am sorry that you are going through these early miscarriages, and hope that the doctor will look into blood tests for you to help reassure you that there is no other issue going on.