Where does “tercio” come from?

tercio (English) comes from Spanish tercio, from Latin tertius, from Latin trēs, from Proto-Italic trēs, from Proto-Indo-European *tréyes.

tercio (English): Any of the three stages of a bullfight: the vara,...

Definitions

Step	Language	Word	Meaning
1	Spanish	tercio	third; third, one of three equal parts; tercio
2	Latin	tertius	third, the ordinal number after secundus and...
3	Latin	trēs	three; 3
4	Proto-Italic	trēs	three
5	Proto-Indo-European	*tréyes	—