How would I "fit" a side to corners?

we might need three triangles

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You just might sadly… I just know that it’s a lot of math…

You can do this by running this in the command bar.
a,b,c,d = table.unpack(game.Selection:Get());

a.Position = b.Position - ((b.Position - c.Position) / 2) - ((b.Position - d.Position) / 2);
a.Size = Vector3.new(0.1, (b.Position - d.Position).Magnitude, (b.Position - c.Position).Magnitude);

If you want to make it into a function, you can do this:
function fit(corner1, corner2, corner3, corner4, wedge)

wedge.Position = corner1.Position - ((corner1.Position - corner2.Position) / 2) - ((corner1.Position - corner3.Position) / 2);
wedge.Size = Vector3.new(0.1, (corner1.Position - corner3.Position).Magnitude, (corner1.Position - corner2.Position).Magnitude);

end

Which you can call like this:
fit(corner1, corner2, corner3, corner4, wedge)