Creating index for specific values only
Database SQL Language Reference - CREATE INDEX


  "Oracle does not index table rows in which all key columns are null
  except in the case of bitmap indexes."


Demo

create table t as select level as field3 from dual connect by level &lt;= 1000000;
alter table t add field3_ind as (case when field3 = 101 then field3 end);
create index t_ix_field3_ind on t (field3_ind); 




select * from t where field3_ind = 101;






select    segment_name
         ,segment_type
         ,bytes

from      dba_segments 

where     owner        =  'DMARKOVITZ' 
      and segment_name in ('T','T_IX_FIELD3_IND')

order by  segment_name             
;




+-----------------+--------------+----------+
| SEGMENT_NAME    | SEGMENT_TYPE | BYTES    |
+-----------------+--------------+----------+
| T               | TABLE        | 13631488 |
+-----------------+--------------+----------+
| T_IX_FIELD3_IND | INDEX        | 65536    |
+-----------------+--------------+----------+


P.s.
The use of virtual column makes the querying more user friendly.  

If the index would have been defined as -

create index t_ix_field3 on t (case when field3 = 101 then field3 end); 


then in order to use the index, the query should have looked like -

select * from t where case when field3 = 101 then field3 end = 101;