'Three' is PIE *treyes — the th- reflects Grimm's Law. Cognate with Latin 'tres' and Greek 'treis.'
The cardinal number following two and preceding four; the sum of two and one.
From Old English 'þrēo' (feminine and neuter; masculine 'þrīe'), from Proto-Germanic *þrijiz (three), from PIE *tréyes (three), one of the most stable words in the entire Indo-European family. The numeral three is attested without significant alteration from Sanskrit to Irish: Sanskrit 'tráyas,' Greek 'treîs,' Latin 'trēs,' Gothic 'þreis,' Old Irish 'trí,' Lithuanian 'trỹs.' The initial /θ/ (th-) in English and other Germanic languages reflects Grimm's Law, by which PIE *t- became Proto-Germanic *þ- (the th-sound), one of the hallmark consonant shifts
The word 'three' is a textbook illustration of Grimm's Law: PIE *t became Germanic *þ (th), which is why Latin 'trēs' and English 'three' begin with different sounds but are the same word. This same sound shift turns Latin 'pater' into English 'father' and Latin 'pisces' into English 'fish.'