Why doesn't this RegionPlot3D not plot anything?

RegionPlot3D[ 0 < Subscript[y, 1] < 1 && Subscript[y, 2] == 0 && Subscript[y, 3] == 1 - Subscript[y, 1] && Subscript[x, 1] == 1 && Subscript[x, 2] == 0 && Subscript[x, 3] == 0, {Subscript[y, 1], 0, 1}, {Subscript[y, 2], 0, 1}, {Subscript[y, 3], 0, 1}]


The result I would expect would be this one:

RegionPlot3D[ 0 < Subscript[y, 1] < 1 && Subscript[y, 2] == 0 && Subscript[y, 3] == 1 - Subscript[y, 1], {Subscript[y, 1], 0, 1}, {Subscript[y, 2], 0, 1}, {Subscript[y, 3], 0, 1}]


I have much more complicated expressions than this, where the common theme is that there will be 3 free variables which I specify where to plot (here, $$y_1, y_2, y_3$$) and all other variables will be given, like above (e.g., $$x_1, x_2, x_3$$ here) and I want to plot the resulting region. Is there a correct way of doing this?

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contrains =
0 < y[1] < 1 && y[2] == 0 && y[3] == 1 - y[1] && x[1] == 1 &&
x[2] == 0 && x[3] == 0;
reg = ParametricRegion[{{y[1], y[2], y[3]},
contrains}, {{y[1], 0, 1}, {y[2], 0, 1}, {y[3], 0, 1}, x[1], x[2],
x[3]}];
DiscretizeRegion[reg, Boxed -> True]
Region[Style[reg, Thick, Blue], Boxed -> True]


• Is there really no way to use RegionPlot3D? I wanted its features, like the labels and all, or other future necessary things. Region seems to be very general, though.
– Jay
Commented Nov 15, 2021 at 6:10
• @Jay Up to the 12.3.1 version, RegionPlot3D still can't draw such equations. We can use label in Region or use Show to add another objects in such figure. Commented Nov 15, 2021 at 6:57