Hi Denis,
Charlie Alquino's solution may work though it will be slow.
If you have 1 record per MBLNR, then this is a unique key. All 3 tables should thus be defined as TYPE SORTED TABLE WITH UNIQUE KEY mblnr.
I did not understand if table 3 is alrteady filled with mblnr an some attributes or if this shall be the result of the process.
For all table accesses use field-symbols. It is faster, i.e.
LOOP AT 3rd_internal_table ASSIGNING <3rd_internal>.
LOOP AT 1st_internal_table ASSIGNING <1st_internal>
* Loop will use the table key - very fast - one record found or none!
WHERE mblnr = <3rd_internal>-mblnr.
<1st_internal>-field1 = <3rd_internal>-field1.
<1st_internal>-field2 = <3rd_internal>-field2.
*...(COPY the values needed from the 1st table to 3rd table).
ENDLOOP.
*...
ENDLOOP.
If you are on ABAP 740 you can use in-line-declaration of field-symbols, i.e.
LOOP AT 3rd_internal_table ASSIGNING FIELD-SYMBOL(<3rd_internal>).
Regards
Clemens