I asked my mother how old I was then .Mother said that I was three years old in that time .

in--at

in that time -> at that time

那时侯我问我妈妈我几岁了。母亲告诉我才3岁 at that time

你要做什么？翻译的话是这样的：
我问我妈妈当时我多大？ 我妈妈说当时我三岁．

并且 应该是 at that time.

我问妈妈当时我多大了
妈妈说我当时3岁了

我问妈妈那时我几岁，妈妈说那时我三岁了