Yes, it's fine.

It seems OK to me as long as you say, almost immediately, "Until that fateful day..." or "But then, after an argument, two of them moved out." The use of 'had' needs resolving quickly. What's the next line?

@OldBrixtonian They had been living there ever since they ended up getting jobs etc.

@OldBrixtonian..this part is actually an intro to the characters....that is the last line of the paragraph.

@Lambie: Yes. That seems fine. What about, 'They had rented a house and had been living there ever since they ended up getting jobs in the same city'?

Is "I rented a flat and had been living in it." OK without a corollary?

@Faj: Lambie's happy with it. I'm not sure about it and I'm hoping others will join in!

Or should I say ' they were living there ever since?

Is this the end of the story, or does something happen after that, perhaps interrupting their living together? If this is the end, this is bad grammar.

@gotube how can I make it right?

A, B, C, and D had known each other since childhood. They went to the same school, and even though they had to go to different colleges, they remained thick friends. As luck would have it, they all ended up getting jobs in the same city. Together, they rented a two-bedroom house and had been living there ever since. This is the para

How can I make it better? Any suggestions??

No...he is trying to introduce the characters in this para...after this, it's a dialog between two of those characters

Is it not clear enough?

A, B, C, and D had known each other since childhood. They went to the same school, and even though they had to go to different colleges, they remained thick friends. As luck would have it, they all ended up getting jobs in the same city. Together, they rented a two-bedroom house and were living there ever since....will this make it right?

Do you mean 'firm friends'? As Old Brixtonian says, they had been living there ever since only works if you go on to say ...until [this] happened or words to that effect. That is, you are setting the scene for the next incident in the story.

A, B, C and D had known each other since childhood. They went to the same school, and even though they attended different colleges, they remained thick friends. As luck would have it, they all ended up getting jobs in the same city after their graduation. Then they rented a two-bedroom house together and started their professional journey three years ago....I changed the para a little bit. How is it now??

You can't use ago unless you are telling the story in the present tense (it means 'three years before now), and you said you were telling it in the past tense. They had been living there for three years when..._ (NB Thick friends is not idiomatic.)

Can I say 'three years before'?

@Faj If you have a single question about the English language, please get answers in the answer section. Please do not use the comment section as a writing clinic. Comments are for notes and questions about the question itself.