it is three years since they -- with each other,and they are

. fell in love
fall in love爱上某人；坠入爱河
a , were in love 意思上是可以,但是侧重于 状态, fall in love 则更侧重于 动作, since 后通常跟表示动作的从句（非延续性）,表示“自从”某件事发生之后