I need to read in CMIP3 datasets in the ESMValTool. I started to implement this feature in the code but it was not successful as I did something wrong or missed something for sure.
Is there anyone who could support me?

What I did:
I modified cmor/table.py and config-developer.yml and added the CMOR table file cmor/tables/cmip3/Tables/CMIP3_A1 (see branch version2_development_cmip3).
And I tried it with adding this dataset lines in the recipes/recipe_python.yml:

  - {dataset: ukmo_hadcm3,  project: CMIP3,  mip: A1,  exp: 20c3m,  ensemble: run1,  start_year: 1997,  end_year: 1999}
  - {dataset: ukmo_hadgem1,  project: CMIP3,  mip: A1,  exp: 20c3m,  ensemble: run1,  start_year: 1997,  end_year: 1999}

On DKRZ the data structure is like this:
<CMIP3-directory>/20c3m/atm/mo/pr/ukmo_hadgem1/run1/pr_A1.nc

The error message is:
esmvaltool._recipe_checks.RecipeError: Unable to load CMOR table 'CMIP3' for variable 'tas' with mip 'A1'