When you want to change a 3D widget material of a 3d widget component in a blueprint, you have to understand how the parameters in this material work. Since i didn’t know this before i had to change one of these materials on my own, i compiled my findings into this small video.
This video covers the importance of the correct naming of the materials parameters.
also i put an example of a changed 3d widget material in a pastebin
Source : https://forums.unrealengine.com/community/community-content-tools-and-tutorials/1785955-working-with-3d-widget-materials