Dropdown list box in selection screen

Hello Expert,

I am creating a selection screen wherein I have to use drop-down list box for 3 fields. I want to get the current field on which the cursor is placed so that its drop down list will be displayed.I tried it using USER-COMMAND fcode option with parameters statement. But the control doesn't go to the AT SELECTION-SCREEN event when we click on a field for displaying its drop down.

How can I get the current parameter to display its dropdown?

Please help.Its urgent.