A triple-to-double coincidence ratio (TDCR) counter was recently constructed at IRA-METAS for liquid scintillation based primary activity standardisations. A description of its optical chamber, efficiency change tools, photomultiplier tubes (PMTs) and electronics is given. This TDCR system was validated by measuring several standard solutions of beta emitters including Ca-45, C-14, Ni-63 and H-3. The activity concentrations, obtained from these measurements and efficiencies computed with a FORTRAN code we developed for symmetric and asymmetric PMTs, agree with the certified values within uncertainties.