Hi -
1. Decalare 3 variables var1, var2 and var3.
2. Read itab1-col1 into var1 ( index 1).
3. Read itab2-col1 into var2 ( index 1).,
4. Read itab3-col1 into var3 ( index 1).,
5. If VAR1 LT VAR2 and VAR1 LT VAR3 then loop at itab1.
6. If VAR2 LT VAR1 and VAR2 LT VAR3 then loop at itab2.
7. If VAR3 LT VAR1 and VAR3 LT VAR2 then loop at itab3.
Hope this helps.
Let us know, in case any issues 
Regards,
Atul Mohanty