I’ve added a ticket for this – if you still need the feature, go vote for it.
I’d love to see this too. When using WordPress as a general purpose CMS, rather than a mere blogs, the amount of metadata, taxonomies and custom fields inevitably increase. The option of a third column to organise them in would be helpful.
What would be extra neat would be the capacity to arrange two columns underneath the content box, like so:
|------------------|-----|
| main content | rhs |
|------------------| |
| col 1 | col2 | |
|---------|--------|-----|
