This would constitute "batch" conversions which the developers state as on of the features of the 3.0 release.
Once again, looks like it wasn't actually ready for release just yet.
The batch input is great, but a batch output - to convert a video 3 (or more) times to different formats for exactly James' reason - would be a godsend. Thanks to all the Miro team and contributors, a brilliant app.
I'm not sure if batch conversion is still a problem, but I do what many others have done, in that I make several copies of the Miro app (and keep them all in my 'applications' folder), open them all and drop the same video file onto each of them, using each app to convert into a different codec. You can also drop multiple files onto each app, as long as you convert them all to the same output codec.
Hope this helps.
I would love if you enable this option. For me as video editor will be very useful to convert files to multiple formats at once, especially when saving files for web distribution.