After finding very little information to help CMS novices like me to get fixed width left and right columns, and having to work it out for myself, I thought I'd share with those who are still struggling with the default vB 4.0.6 grid layouts being insufficient for your needs.
The key is to download the default grids.xml to see how they're made: Grid Manager > Download/Upload Grids.
I needed both the right and left columns to be of fixed widths (180px & 240px) and with a fluid middle column (100%). See the code below to see how easy it is to accomplish yourself:
You just need to save the above code as an .xml document and upload using the Grid Manager > Download/Upload Grids. Make sure that you name the new grid.xml clearly so that you'll recognise it in the Grid Manager. I named the above grid "Sidebar + Column + Sidebar 180px, 100%, 240px.xml".
I've attached the above grid as an .xml document for your convenience.