I'll try to give you some kind of answer - I don't think what you're proposing is all that difficult, but to be honest I'm not 100% sure it can be done using the plugin system. If you don't mind modifying the vb files then it shouldn't be too difficult. (I know that's still kind of vague).
But what about this: just make 3 single selection menus fields with the same options in each, then you will automatically have exactly 3 selections in a given order, and you only have to edit your postbit template and put in {vb:raw post.fieldX}, {vb:raw post.fieldY}, and {vb:raw post.fieldZ} (or use those values to build image names for example). Edit: Oh, well, I guess that would allow the same value to be selected twice.