In 1 Timothy 4:1-3, the apostle Paul speaks of the apostasy of the church that would occur in the last days of the apostles. After the apostles had died, the doctrine of the trinity was one of the false doctrines that arose.
There is no formal disproof of the trinity doctrine in the Scriptures because the trinity was never mentioned or taught in the Scriptures. Again, it came afterward. You just have to rely on the Scriptures insistence that God is one (Deuteronomy 6:4), your common sense, and the witness of the Holy Spirit in your conscience. Conversely, people who cling to the trinity put their trust in church, which, since the church is composed of people, is antithetical to trust in God (Proverbs 29:25 and Jeremiah 17:5-8).