Seems to me you need multiple List controls in an HBox, in this case three List controls. That way you can get the desired behavior.
You may need to create three dataProviders.
A List class like TileList does a couple of important things. It only creates enough renderers to fill the viewable portion so you don't end up allocating tons of memory for 1000's of data items, and it has a selection model where you can select one or more data items and it draws a rollover and selection colored backgrounds over the entire renderer.
Do you need that? If not, a VBox with 3 HBoxes with 1/3 of the checkboxes in each HBox is all you need. You can use a Repeater to generate the checkboxes or just add them in a loop.
Flex SDK Developer
Adobe Systems Inc.