problem in selection screen

Hi,

I have created two screens and called one after another with some usercommands at standard selection screen.When I press enter or execute at the third screen it comes back to the 1st screen(select screen-1000).But I do not want this.Please help.Following is my code.

SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME TITLE text-001.

PARAMETERS: a TYPE xxxx ,

b TYPE xxxx,

c TYPE xxxx .

SELECTION-SCREEN END OF BLOCK b1.

*Selection screen for 2nd screen

SELECTION-SCREEN: BEGIN OF SCREEN 2000 TITLE text-017 .

SELECTION-SCREEN BEGIN OF BLOCK b2 WITH FRAME TITLE TEXT-016.

PARAMETERS: pdir type STRING OBLIGATORY DEFAULT TEXT-020.

SELECTION-SCREEN PUSHBUTTON 79(10) but1 user-command CLI1.

SELECTION-SCREEN END OF BLOCK b2.

SELECTION-SCREEN:END OF SCREEN 2000.

SELECTION-SCREEN: BEGIN OF SCREEN 3000 TITLE text-017 .

SELECTION-SCREEN BEGIN OF BLOCK b3 WITH FRAME TITLE TEXT-018.

PARAMETERS: pdir1 type STRING OBLIGATORY DEFAULT TEXT-020.

SELECTION-SCREEN PUSHBUTTON 79(10) but2 user-command CLI2.

SELECTION-SCREEN END OF BLOCK b3.

SELECTION-SCREEN:END OF SCREEN 3000.

AT SELECTION-SCREEN .

CASE sy-dynnr.

WHEN 1000.

CASE sy-ucomm.

WHEN c_create.

PERFORM create.

WHEN c_push.

PERFORM show.

WHEN c_execute.

CALL SELECTION-SCREEN 2000.

WHEN c_back OR c_end OR c_canc.

LEAVE PROGRAM.

ENDCASE.

WHEN 2000.

CASE sy-ucomm.

WHEN 'CLI1'

PERFORM load.

WHEN c_execute.

PERFORM update

CALL SELECTION-SCREEN 3000.

ENDCASE.

WHEN 3000.

CASE sy-ucomm.

WHEN execute.

PERFORM download.

ENDCASE.

ENDCASE.