"Tingkat investasi dari Jepang turun sejak tiga tahun lalu."

Translation: The investment level from Japan has been declining since three years ago.

[donny309620]

"The investment level from Japan has been declining since 2016" ("since three years ago" doesn't sound quite right, in this case, it's better to nail down the approximate or definite date, if known) or, another way of to say it is: "The investment level from Japan has been declining 'for the last' three years."

[tsuj1g1r1]

Yup, “since ... ago” = “for ...” in English.

[dkfour]

"The level of investment from Japan has been declining for three years now."