Welcome to HCM.
You have not mentioned the dates on which you did your pregnancy
tests. If the interval between 3rd and 4th test was 1 Week or more, the possibility of missed abortion
has to be entertained.
If the interval was of only 2 or 3 days, then most likely the 3rd test was false positive.
If you have not yet got your regular period, best would be to get an ultra sound