SQL> ed
Wrote file afiedt.buf

  1  with ist_dlgn (ln,naimFull,dateBegin, dateEnd, pos) as (
  2  select 1,'Aleks27',date '2018-01-01',date '2018-02-01','R' from dual union all
  3  select 1,'Aleks27',date '2018-02-01',date '2019-01-01','R' from dual union all
  4  select 1,'Aleks27',date '2019-01-01',date '2020-02-01','R' from dual union all
  5  select 1,'Aleks27',date '2020-02-01',date '2020-03-01','R' from dual union all
  6  select 1,'Aleks27',date '2020-03-01',date '2020-04-01','K' from dual union all
  7  select 1,'Aleks27',date '2020-04-01',date '2020-05-01','O' from dual union all
  8  select 1,'Aleks27',date '2020-05-01',null,'R' from dual union all
  9  --
 10  select 2,'Stax',date '2021-04-01',date '2021-05-01','O' from dual union all
 11  select 2,'Stax',date '2021-05-01',null,'R' from dual
 12  )
 13  select * from (
 14    select *
 15    from ist_dlgn i
 16    where
 17        ln=1
 18      and dateEnd is not null
 19      and pos<>'K'
 20    order by dateBegin DESC
 21  )
 22  where ROWNUM<=3
 23* order by dateBegin
SQL> /

        LN NAIMFUL DATEBEGIN  DATEEND    P
---------- ------- ---------- ---------- -
         1 Aleks27 01.01.2019 01.02.2020 R
         1 Aleks27 01.02.2020 01.03.2020 R
         1 Aleks27 01.04.2020 01.05.2020 O

SQL>

Аналитические функции, появившиеся в версии Oracle 8.1.6, создавались для реше-
ния именно этих задач.

with ist_dlgn (ln,naimFull,dateBegin, dateEnd, pos) as (
select 1,'Aleks27',date '2018-01-01',date '2018-02-01','R' from dual union all
select 1,'Aleks27',date '2018-02-01',date '2019-01-01','R' from dual union all
select 1,'Aleks27',date '2019-01-01',date '2020-02-01','R' from dual union all
select 1,'Aleks27',date '2020-02-01',date '2020-03-01','R' from dual union all
select 1,'Aleks27',date '2020-03-01',date '2020-04-01','K' from dual union all
select 1,'Aleks27',date '2020-04-01',date '2020-05-01','O' from dual union all
select 1,'Aleks27',date '2020-05-01',null,'R' from dual union all
--
select 2,'Stax',date '2021-04-01',date '2021-05-01','O' from dual union all
select 2,'Stax',date '2021-05-01',null,'R' from dual 
)
select * from (
  select i.* 
   ,row_number() over (order by dateBegin DESC) rn
  from ist_dlgn i 
  where 
      ln=1
    and dateEnd is not null 
    and pos<>'K' 
--  order by dateBegin DESC
) 
where RN<=3
order by dateBegin
SQL> /

        LN NAIMFUL DATEBEGIN  DATEEND    P         RN
---------- ------- ---------- ---------- - ----------
         1 Aleks27 01.01.2019 01.02.2020 R          3
         1 Aleks27 01.02.2020 01.03.2020 R          2
         1 Aleks27 01.04.2020 01.05.2020 O          1

SQL>

select * from (
  select i.* 
  ,(select count(*)
    from ist_dlgn ii 
    where 
          ln=1
      and dateEnd is not null 
      and pos<>'K' 
      and ii.datebegin>=i.datebegin
  ) rn
  from ist_dlgn i 
  where 
      ln=1
    and dateEnd is not null 
    and pos<>'K' 
)  where rn<=3
order by datebegin