using three custom fields for a date, but how use these for "orderby"?!

I have three custom meta fields in a custom post type, one for date, one for month, and one for year. The two first selectable by a drop-down, the last a normal textfield.

Earlier I have used one text field, where the user had to write the date as "YYYYMMDD" (but this was obviously asking too much...), and then used this to order by using metakey/value and so on.

So... is there any way I can use those three new fields to order by instead?