"since 3 years" or "for 3 years"?
I've lived here since three years.
I've lived here for three years.
The rule
Use "for" with a length of time (for 3 years) and "since" with a starting point (since 2020). "Since 3 years" mixes the two.
"For" and "since" both talk about time, but they answer different questions. "For" tells you how long something lasts (a duration); "since" tells you when it started (a point in time). Mixing them up gives the very common "since 3 years."
More examples
She's worked here for 2020.
She's worked here since 2020.
We've been waiting since two hours.
We've been waiting for two hours.
They've known each other for childhood.
They've known each other since childhood.
How to remember it
for + a length of time (for two hours, for years). since + a starting point (since Monday, since 2020).
Frequently asked
When do I use "for"?
With a period of time: for five minutes, for three years, for a long time. It answers the question "how long?"
When do I use "since"?
With the point when something started: since Monday, since 2020, since I was a child. It answers "starting when?"