if the number fits in with what the site is about I would think it is ok... for example if you were creating a site teaching intro snorkeling tips, you could use something like snorkleing101.com but generally it is best to avoid using numbers whenever possible.
If you are looking to drive mostly organic traffic then three keyphrases with dashes is acceptable if you plan on doing some additional seo work on the site. It still currently doesn't rank as well as exact URL matches though and it will never be easy to market offline or through word of mouth if that is your plan.
I would make sure I've exhausted all non dash, non number solutions first... find some good adjectives to surround your main keyphrase and see if those combos are available before using numbers or dashes.