Well done on your results, that's amazing!
You could call Manchester and tell them your grades and see what they say, if they definitely need a fourth AS then there's no need to apply, forget them!
Your work experience shouldn't matter too much, it depends on your personal statement greatly. You should think about what qualities a dentist needs and then see if any of them have been gained in any work experience. I am applying for midwifery this year but I've done work experience in schools, but I thought about what skills I gained from that and just linked it to midwifery, for example using good communication skills!
Hope this helps!