with z as (
select 'Иванов'  emp, 32 procent from dual union
select 'Петров'  emp, 28 procent from dual union
select 'Сидоров' emp, 21 procent from dual union
select 'Арбузов' emp, 10 procent from dual union
select 'Яблоков' emp,  6 procent from dual union
select 'Грушев'  emp,  3 procent from dual),
 zz as (select emp, procent, 
               row_number() over (order by procent desc) rn
          from z 
  )
select     
      case when rn <= 3 then emp else 'остальные' end As emp,
      sum(procent) sum_pct
  from zz
 group by 
    case when rn <= 3 then emp else 'остальные' end,
    case when rn <= 3 then rn else 4 end;

EMP          SUM_PCT
--------- ----------
Петров            28
Иванов            32
Сидоров           21
остальные         19