[en] The BiPo-3 detector is a low radioactive detector dedicated to measuring ultra-low natural contaminations of "2"0"8Tl and "2"1"4Bi in thin materials, initially developed to measure the radiopurity of the double β decay source foils of the SuperNEMO experiment at the μBq/kg level. The BiPo-3 technique consists in installing the foil of interest between two thin ultra-radiopure scintillators coupled to low radioactive photomultipliers. The design and performances of the detector are presented. In this paper, the final results of the "2"0"8Tl and "2"1"4Bi activity measurements of the first enriched "8"2Se foils are reported for the first time, showing the capability of the detector to reach sensitivities in the range of some μBq/kg. - Highlights: • BiPo3 : low radioactive detector to measure ultra-low levels of "2"0"8Tl and "2"1"4Bi. • Backgrounds characterized. • Surface background : A("2"0"8Tl) =(0.9±0.2) μBq/m"2 and A("2"1"4Bi) =(1.0±0.3) μBq/m"2. • First SuperNemo double β source foils measured : A("2"0"8Tl) =(21±11) μBq/kg and A("2"1"4Bi) < 290 μBq/kg at 90% C.L. • Shows capability to measure "2"0"8Tl in the range of some μBq/kg.