“三年中，他他多次写信给我，鼓励我多交朋友，告诉我学好英语的方法。”这句话翻译成英文用什么时态？

我觉得过去完成进行时或者现在完成进行时都可以：
He has been writing to me many time/several times to encourage me to make friends and teach me the method of learning English during the 3 years. （这个事情现在还在继续，他还在继续给你写信）

He had been writing to me many time/several times to encourage me to make friends and teach me the method of learning English during the 3 years. （这个事情已经结束了，表明他就写了这3年，现在已经没再写了)

several 和 many 视实际次数而定

个人意见哦，仅供参考

用现在完成时。
In/During the past three years / Over the three years, he has frequently written letters to me,encouraging me to make more friends and telling me the way to learn / of learning English well.

"Three years, he wrote to me many times, encouraged me to make more friends, tell me the way to learn English well.