You do realize that just because a data table returns more columns than you need, you don't have to use all of them? If displaying in a datagrid, you can select to hide the columns not needed. As for what three columns you need, you didn't specify anything useful other than demanding a solution.
Do you need the top three rows, the bottom three rows, three rows based on some other criteria? For loops with get you the top or bottom three rows. A query will get you anything in between. Take a look at the TWX help and search for "query". Then use the snippets for the QueryDatatableEntries service.
Next time, perhaps, you provide a bit more info related to what you're specifically trying to accomplish and share what you've already tried. You'll definitely get more people to respond.
