SQL & PL/SQL

I need to split some text strings into columns, using the underscore as a delimiter.

I've got so far, via:

GET sampledata
WITH sampledata AS
     (SELECT 'TEST_PPL_AM_POR' test_key
        FROM DUAL
      UNION ALL
      SELECT 'TEST_CTY_GL_POR'
        FROM DUAL
      UNION ALL
      SELECT 'TEST_CF1_AP_POR'
        FROM DUAL)
SELECT test_key
     , LENGTH(test_key) v1
     , SUBSTR(test_key,0,INSTR(test_key, '_')-1) v2
     , SUBSTR(test_key, INSTR(test_key, '_') + 1, INSTR(test_key, '_') - 2) v3
  FROM sampledata;

TEST_KEY                V1 V2              V3             
--------------- ---------- --------------- ---------------
TEST_PPL_AM_POR         15 TEST            PPL            
TEST_CTY_GL_POR         15 TEST            CTY            
TEST_CF1_AP_POR         15 TEST            CF1         

I can get the first 2 parts split into columns, but then get rapidly confused with the nested INSTRs.

I wondered if there is a simpler route / a better way of doing this?

Any advice much appreciated,

Thanks!