Well, first of all, it's difficult for a person to know size accurately without a scan.
Are you saying that the length is 1.5 cm to 2 cm? 1.5 cm might still be possible from being "reactive", that'd be at the upper limit. Over that size, the only way I know of for a node to be longer while still being non-malignant is from granuloma.
So next we'd want to know: is that node long and thin or rounded? You'd said it is "thin". That tends to being non-malignant. But on the other hand, being rounded tends to being malignant because cancer tends to grow in all directions equally.
Next we'd look at the overall behavior. Nodes popping up while other nodes reduce in size is not how cancer would typically behave. But that is how immune reactions can behave.
"can they be swollen for 3 weeks just because my wisdom tooth descends without infection"
All of your experience is unusual but doesn't seem like cancer. Overall it seems to me that there is some unusual bacteria involved --- or that you have an unusual immune system and so I'd wonder if you have a family history of that.
Since teeth are involved, look here:
"if no what can cause them to be like that for so long?"
Besides existing infection or fibrosis, we can look to more rare and mysterious processes of immune signalling. This becomes very complicated. The body uses many interacting biochemicals. Or even dead bacteria can set off a strong reaction. Or sometimes signalling cells such as macrophages can be overactive and cause problems.
So its all guesswork, but it seems that something was already present and that the descending tooth intensified things.
The length of time (3 weeks) should not be overly alarming and not lead to an automatic conclusion of cancer.