|x-3|
If x is less than 3, will |x-3| be negative? I don't think so.
For example, if x=2, |x-3|= |2-3|=1. x-3 can never be negative, I think.
In this link, it has been said that when x<3, x-3 is negative, thus |x-3|=-(x-3).
I think this statement is totally wrong.
Please let me know what the correct is.

$|x|$ is non-negative regardless of any sign. Absolute value is the distance of a number from $0$ and distance can never be negative!