因为It is three years 在口语中相当于
It has been three years （已经有三年了）,属于现在完成时,所以它的从句要用since引导.
It has been three years since we came to study in our school.
意思：自从我们来到学校学习,已经有三年了.
【提醒一下,there 打错了,应该是three】