A primitive root perhaps originally to intensify, i.e. Treble; but apparently used only as denominative from shalowsh, to be (causatively, make) triplicate (by restoration, in portions, strands, days or years) -- do the third time, (divide into, stay) three (days, - fold, parts, years old).
see HEBREW shalowsh
H8027. shalash
I. [שִׁלֵּשׁ] Pi`el denominative do a third time, devide into three parts, etc.; — Perfect2masculine singular וְשִׁלַּשְׁתָּ֫ Deuteronomy 19:3 divide land into three parts (so Imperfect אֶתהָֿעָם וַיְשַׁלֵּשׁ [for וַיְשַׁלַּח ᵑ0] 2 Samuel 18:2 ᵐ5L Bu); 1 Samuel 20:19 stay three days (compare Dr); 20:20 We Dr and others (compare ᵐ5) read בַּ ׳אֲשַׁלֵּשׁ (for הַ ׳שְׁלשֶׁת) = I will shoot on the third day with the arrows (HPS question); Imperfect3masculine plural וַיְשַׁלֵּ֑שׁוּ 1 Kings 18:34, and Imperative masculine plural שַׁלֵּ֑שׁוּ 18:34 do a third time.
Pu`al Participle מְשֻׁלָּשׁ three years old Genesis 15:9 + 1 Samuel 1:24 (read with ᵐ5 Th We etc. מְשֻׁלָּשׁ בְּפַר for שְׁלשָׁה בְּפָרִם), feminine מְשֻׁלֶּשֶׁת Genesis 15:9 (twice in verse); הַמְשֻׁלָּשׁ הַחוּט Ecclesiastes 4:12 the threefold cord; female plural מְשֻׁלָּשׁוֺת Ezekiel 42:6 three-storied chambers.