Would three negative d dimers miss a clot ?

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

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.

Depends but unlikely: dDimer is an inflammatory response marker. The sensitivity is in the upper 90's for acute thrombo-emboli. If the clot is old it may not generate the response and thus a positive test. The concern about colts is if they get bigger or propagate. Old clots do not often do this. There is no clinical indication to repeat a dDimer as if you have a high clinical suspicion you would do the test.