Hello,
Use these links..
http://scn.sap.com/thread/1694646
http://wiki.sdn.sap.com/wiki/display/ABAP/Dynamic+Internal+table
now 2 point will be cleared and for 3rd one.
Loop 1st Itab and inside loop read 2nd itab and if sy-subrc = 0 delete the data from same 2nd itab using same WA which is for 2nd itab.
BR
Chandra..