Phil, it should be a simple matter of
1. setting the number of items you want per line in the configurations setting in Admin

2. Setting the percentage width each item takes up in the style sheet.

3. Checking to see how wide your items are, in particular the setting on picture widths, and add them up against how wide your overall site and middle content area is. In other words if you have item pictures set to 150 px each, and want three per line, your middle content area better accommodate at least 450 pixels with NO padding or margin per item... otherwise the third item will wrap down to the next line.

JSC, Boulder