The trio of a color and its two enemies is called a wedge because of the shape their triangle makes in the pentagon.
Abzan
Black Green White
We Will Survive by Mark Rosewater
Temur
Blue Red Green
What Doesn't Kill You Makes You Stronger by Mark Rosewater
Mardu
White Black Red
Color Pie Friday: The Philosophies of White/Black/Red by Sarpadian Empires vol. VIII
Finishing First by Mark Rosewater
Mardu Vehicles by Andrew Jessup 4/29/17
Jeskai
Red White Blue
Smart Thinking by Mark Rosewater
Sultai
Green Blue Black
Whatever It Takes by Mark Rosewater