Would three negative d dimers miss a clot?

D dimer. The test is good but not perfect. Most studies show that if used appropriately, it misses a clot less than 10% of the time.
Unlikely. If the d dimer test is negative three times, highly unlikely to miss a clot. An ultrasound could help if you and your doctor are still debating the issue.
Not usually. D-Dimer is a very sensitive (overly sensitive) test for clotting. IF it is persistently negative the risk of a clot is less than 5%. However it also must be combined with your clinical picture together they are used to calculate a "Wells score" . THe most sensitive test for a clot in the leg is a duplex scan but with a normal exam and negative d-dimer it is not likely to be positive.