An agent requests the first 3 chapters of your novel. Simply for the telling of the story, you send 1, 2 and 12. How great a sin has been committed?
The first rule is ALWAYS, without exception: follow the directions.
Agents don't make this stuff up to drive you crazy or to be obstreperous. We WANT to sell your work! Most editors want to read the first three. If anything, send the first three and then include 12 if it provides information needed in the story--like the hero dies and the POV changes or something.
One of the things that makes me crazy is the sense I get from some query letters that "the directions are for other people; they don't really apply to me cause I'm special" and "if you will just read 1, 2, and 12 you'll see why the directions don't apply."
You're not special. I don't care what Mom said. Until I sell your book and we make some money you're a debit on my balance sheet. That's not what you want to hear, but it's the truth. We won't ever say it to your face but it's true nonetheless. You can be an acceptable debit by writing well and being easy to work with. You can be an unacceptable debit by having good writing and being too difficult to work with. Where that demarcation line is, is different for every agent, but every agent has one and you'll be a fool to find out where it is this early.
So, it's not a mortal sin but you're headed in the wrong direction.
Follow the directions.