If it's stated in UCAS points you will still get in, if it's actually written down as CCC (and doesn't say something like "or equivalent") then it is very much down to their discretion, and will depend not only on your grades, but also the grades of other people who apply. It also depends sometimes on which grade the D was in. If it's English then maybe not!
The best thing to do would be to get in touch, as they may let you in anyway .