It has been three months since we started this term.
It is three months since we started this term.
You can use present tense. Think of it this way:
Today it is 3 months since the term started.
It is a week since I saw him.
Because the term has a fixed start date, I prefer to say �since the term started � rather than �since we started �
It has been 6 months since his accident, but his knee is still painful.
It is 6 months since since his accident, but his knee is still painful.
Both these sentences are correct. The usage depends on the context.
How long is it since he had his accident? It is six months.
How long has it been since his accident? It has been six months.