There are a whole host of things that arise from your post.
At the moment you will get an A level grade based on just your AS units so it will be terrible. You need to consider the implications of this - talk to the centre.
Did you tell UCAS you were taking 4 A levels? If so, you need to tell them that you are only taking 3.
Did you get an AS certificate last year (and not re-sit an AS unit this year)? If yes, then there is no need (nor can you) cash in the AS again.
Talk to your centre ASAP - just not turning up will have all sorts of consequences. Make sure you understand the consequences of what you're proposing - or just revise like mad and give it a go.