"Mereka tinggal bersama kami sejak tiga tahun yang lalu."

Translation: They have lived with us since three years ago.

[Syahira372075]

"For the past three years, they have lived with us" is a more commonly used phrase

[KyrieDomineJesus]

Correct:

"They have lived with us since 2017."

"They have lived with us for 3 years."

"They came to live with us 3 years ago."

Wrong:

"They have lived with us since 3 years ago."

"They have lived with us since 3 years."

[AnitaRRC]

SINCE is used with a gixed date or time. FOR defines a period.
https://www.grammar.cl/Notes/For_Since.htm

[AnitaRRC]

Here it could be Ria's sentence or they came to live with us 3 years ago if you want ago. Since would need 'since December 2015'

[Kst868213]

I agree with you guys, this sentence sounds wrong to me too.

[dkfour]

"have lived... since" or "lived... for"

[Haakon388572]

The combination of "since" and "ago" in the same expression is not good English.