It was three months since we had been here. （到那时）我们已经有3个月没有来这里了。【或者：（到那时）我们不在这儿已经三年了。】

It is three years since I have seen Bill. ( = It is three years since I [last] saw Bill.) 我已经有3年没有见到比尔了。（即：自从我上次见到比尔到现在已经有3年了）

It is three years since I have seen Bill. ( = It is three years since I [last] saw Bill.) 我已经有3年没有见到比尔了。（即：自从我上次见到比尔到现在已经有3年了）（see是非延续性动词）

It is two months since Tom has smoked a cigarette. (= It is two months since Tom [last] smoked a cigarette. )汤姆已经有两个月不抽烟了。（smoke 是延续性动词）

好的明白了，谢谢黎老师。