Assuming your report has three levels of grouping (Section, SubSection and Detail) then:
- In the Section Header include a text box with a name 'txtSectionNo', and has as its control source '=1', and set the Running Sum property to 'Over Group'
- In the SubSection Header include a text box with a name 'txtSubSectionNo', and has as its control source '=1', and set the Running Sum property to 'Over Group'. Set the Visible property for this text to False
- In the SubSection Header include a text box with a name 'txtDisplaySubSectionNo', and has as its control source '=[txtSectionNo] & "." & [txtSubSectionNo]'
- In the Detail section include a text box with a name 'txtSubSubSectionNo', and has as its control source '=1', and set the Running Sum property to 'Over Group'. Set the Visible property for this text to False
- In the Detail section include a text box with a name 'txtDisplaySubSubSectionNo', and has as its control source '=[txtSectionNo] & "." & [txtSubSectionNo] & "." & [txtSubSubSectionNo]'