I always thought it worked this way:
You can put in up to 18k of your own money, and the percentage the company matches (3% in my case, + 5% profit sharing) is in addition to that 18k.
However, someone recently told me that the 18k is a limit for the combination of your contributions AND the employer's.
I am hoping that I was right, as my auto contributions added up to about 18k.
Thanks