On the Home tab of the ribbon, in the Paragraph group, click the drop-down arrow on the right of the Numbering button.
Select 'Define New Number Format...'
In the resulting dialog, select 1, 2, 3 from the 'Number style' dropdown.
In the 'Number format box, make sure that you don't remove the number 1.
Click before it and insert an opening parenthesis (
Click after the number, remove the point and insert a closing parenthesis )
Click OK.