select   (same_date/days_3_back)*100 as percent_change
from(
      select   sum( case when order_dt = to_date(sysdate,'DD/MM/YYYY')
                    then 1
                    end 
                  ) as same_date,
               sum( case when order_dt = to_date(sysdate - 3,'DD/MM/YYYY')
                    then 1
                    end
                  ) as days_3_back
      from     source_table
)

SQL> select to_date(sysdate,'DD/MM/YYYY') from dual;
select to_date(sysdate,'DD/MM/YYYY') from dual
               *
ERROR at line 1:
ORA-01830: date format picture ends before converting entire input string

with t as (
           select  count(*) over(order by trunc(order_dt) range between current row and current row) order_count_this_day,
                   count(*) over(order by trunc(order_dt) range between 3 preceding and 3 preceding) order_count_3_days_back
             from  source_table
          )
select  case
          when order_count_3_days_back != 0 then order_count_this_day * 100 / order_count_3_days_back
        end percent_change
  from  t
/