its not the case that universalists usually reject the 'Trinity'. In fact, the so-called church-faters who developed the doctrine (the term itself was only hammered out in the 4. century, but the concept has been present from the beginning of the early church.) believed all in Universal Salvation:Trinitarian Universalism
Personaly I dislike the term 'Trinity' and prefer the term Family or Community. I think that's far more scriptural. Also I think it is more about the knowledge of Father's Heart than about correctly dividing the Deity. But I had some God-encounters in the last few months that led me to a deeper study on this subject, which has absolutley convinced me of the triune nature of God.
UR was for me never about a doctrine but about Father's Love and my experience of it has led me to the place where I had to embrace the idea of Perichoresis
. Sadly many people reject the idea of the 'Trinity' without actually really knowin what it means.
There have alway been different definitions of it from the early days on- especially between the western and the eastern branch of the early church. The reason for it was that the westerners saw it- influneced by their roman culture- more as hierachical, while the easterners were more mystic in their approach.The early church went down a dangerous path when they placed explanations of Father, Son and Holy Spirit above experiences with Him.
This website has some good information on why the early christians insisted on the Eternal Nature and Unity of Father, Son and Holy Spirit
and how they had to fight greek philosophy, roman law and jewish prudency to stick to their conviction of God's Nature.
Whatever you study always depend on the Holy Spirit, He is your Teacher and He will lead you into all truth.