◆ His brother has been in the army for three years.
◆ His brother has been in the army since three years ago.
◆ His brother has been a soldier for three years.
◆ His brother has been a soldier since three years ago.
◆ His brother joined the army three years ago.
◆ It is three years since his brother joined the army.
◆ Three years has passed since his brother joined the army.