This is a proof of concept how to expand the 'perfect magma trap' by Kanddak to a 3 tiles wide floor. Refer to http://mkv25.net/dfma/movie-1808-perfectmagmatrap for the general concept.
For the test I didn't use magma but water, and I left out the automation system of the original.
The problems to solve where the following:
1. Floor grates have to be connected to walls or floors and cannot support eachother. 3x3-grate floors are impossible, so you'll need a floor somewhere. A 1/7 tile of fluid could be left behind there exposed to evaporation.
The solution is a pump one level over the non grate tile that removes the fluid there. This is expandable in length when you add a floor-tile and a pump every three tiles.
2. The pumps at the lowest level have to be powered from one level beneath, so the the tiles next to it cannot be dug out. Anoter place for possible evaporation.
The solution is a big enough reservoir to leave behind fluid with a depth of at least 2/7 everywhere.