Evrrytime I think I understand it turns out to be a heresy

average peasant during the reformation actually reading the bible for the first time

Sure! So first, even though historically Christians would affirm the Nicaean Creed which espouses the trinity, not all Christians adhere to the trinity. Unitarians and Christadelphians, for example, along with a lot of Pentecostals. These are non-Nicenean Christians which may or may not be considered Christians depending on who you ask.

Some history. The Trinity developed alongside the canon, or which books did and did not make it into the New Testament. Philosophical discussions that leaned heavily on the Greek philosophy schools became either the foil or the leader in the arguments. So Gnosticism and similarly Docetism was rejected because Jesus was a real human which, in turn, led to the Gospel of Thomas being rejected. But Jesus was also considered divine, so the ongoing debates focused on how to reconcile God being human and also divine and also still God. This wasn't a quick process. It also caused huge divisions with the Eastern church rejecting the Filioque clause; the argument was/is largely over where the Holy Spirit comes from and its relationship to Jesus, but it's now just about tradition as its so deeply rooted at this point. So the trinity is not just answering "What is the nature of God" but also "How does that affect free will, faith, creation, etc." which means all explanations turn into a huge tome. As cultures change, explanations change and are accepted or rejected based on philosophic and scientific shifts. They all are attempting to explain how God can remain outside of time as "father" and inside of time as "spirit" and Jesus' relationship to both.

Even though some groups may act like nothing has changed since the fourth-century CE, there were many, many different phases in the development and understanding of the trinity. The famous allegories pop up early on and are sometimes praised at the time and later rejected (especially in the medieval period). This is very different from, say, Karl Barth. His dialectical understanding of the trinity in which we approach the I-Thou distinction by eliminating the differences God that acts in and through history for salvation ("economic trinity") and the eternal God outside of time ("immanent trinity"). Put another way, the relation between "God in Godself" and "eternal God and temporal acts." Barth is interested in how the eternal can enter time, how the divine can become flesh and still remain divine, etc., and that eventually makes him create such a picture of the trinity. This was a huge shift at the time. There will be more to come, for sure.

All that to say: don't worry about being confused about the trinity. Every generational of people have a go at it and try to understand it in their own culture, place, and time. As always, these things shift and change as material conditions shift and change. Find an explanation that has resonance with you, even if just for logical explanations. You may find "heretical" explanations are better than historical ones!

Two books that offer good introductory articles to guide you if you want to explore more: Cambridge Companion to the Trinity (2011) and Oxford Handbook of the Trinity (2012). If you only have 10 mins, read the preface/intro to each and find a chapter that sounds interesting.