There is a hacky way to plot a set of polygons. The main idea is to merge everything into a single list, manually “close” each polygon by repeating the first point, and “restart” the polygon drawing by inserting an undefined value between each.
In your graph, I joined (A,B,C,(0/0,0/0)) and manually re-indexed so that it would connect [a1,b1,c1,undefined; a2,b2,c2,undefined; …]: desmos.com/calculator/s0feftgikl
Here’s the same idea, but with the potential for an arbitrary number of triangles: desmos.com/calculator/a6lsmnwit4.
The calculator still thinks you are trying to draw a single polygon, though, so the fills can sometimes intersect with one another and “undraw” portions. If you define your coordinates manually, you can choose a consistent orientation to connect the dots and should be able to avoid this.