column new_str format a30
with t as (
           select 'Net Amount Payable by an Individual' str from dual union all
           select 'Net Amount Payable by an Individual+Tax' from dual union all
           select 'Total Amount Payable towards Service' from dual union all
           select 'Total Amount Payable towards Service.+Tax' from dual
          )
select  str,
        rtrim(regexp_replace(str,'(\w{1,3})\w*(\W+|$)','\1_'),'_') new_str
  from  t
/

STR                                       NEW_STR
----------------------------------------- ------------------------------
Net Amount Payable by an Individual       Net_Amo_Pay_by_an_Ind
Net Amount Payable by an Individual+Tax   Net_Amo_Pay_by_an_Ind_Tax
Total Amount Payable towards Service      Tot_Amo_Pay_tow_Ser
Total Amount Payable towards Service.+Tax Tot_Amo_Pay_tow_Ser_Tax

SCOTT@orcl >