Explanation

Here three years indicates past tense.

First clause belongs to Past Continuous Tense.

We use verb + ing in this Tense

Three years ago was studying in London, now I study in Pakistan.

تین سال پہلے لندن میں پڑھتا تھا، اب پاکستان میں پڑھتا ہوں۔

agm.5.3.23