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It's my understanding that the three are fused together in an attempt to alleviate the issue of dust getting caught. Meaning you need to replace the lot. I've just lived with the crack in mine.
 
I have also heard that the glass and lcd are fused. Not sure why they are selling just the glass.

Unfortunately, ifixit.com confirms this.

Quote from the site:

If you bought them separately, be careful when sticking the LCD to the digitizer as the entire surface of the LCD is covered with glue. Once it sticks to the LCD, you will not be able to remove it.
 
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