/u/solotopplaneonly
The origins of the "Tercio" has been discussed here several times, including by /u/EmperorCharlesV himself here. Joking aside, the term "Tercio" can mean two things, either the way of fighting by, or the organization of, armies. Oftentimes it means both.
Charles V was the first to refer to "the tercio of Naples and Sicily, the tercio of Lombardy, and the tercio of Malaga" in an ordinance promulgated in 1536.
As to how they developed, that's a complex question. You should get started by reading some great questions by /u/hborrgg on /r/EarlyModernEurope, such as this one. I think some threads there contain the best references in English language. If you want more critical analysis as to "why this and not that" then you should look at the list of references of that last thread link. And then ask specific follow up questions here.
Cheers.