Combine multiple swatch legends

Question

How can I combine multiple SwatchLegends? As a minimal example:

Legended[
  Show[g1 = Graphics3D[{Opacity[0.6], Red, Sphere[]}]], 
    SwatchLegend[{Red, Red, Red}, {"x\[Rule]ξ", "y\[Rule]η", "z≡ζ"}]]

Legended[
  Show[g2 = Graphics3D[{Opacity[0.4], Blue, Cylinder[]}]], 
    SwatchLegend[{Blue, Blue, Blue}, {"ξ≡ξ'", "η\[Rule]η'", "ζ\[Rule]ζ'"}]]

Legended[
  Show[g3 = Graphics3D[{Green, Cone[]}]], 
    SwatchLegend[{Green, Green, Green}, {"ξ'\[Rule]x'", "η'\[Rule]y'", "ζ'≡z'"}]]

Now I would like to combine them; something like

Legended[
  Show[{g1, g2, g3}], 
    SwatchLegend[{
      {Red, Red, Red}, {"x\[Rule]ξ", "y\[Rule]η", "z≡ζ"}, 
      {Blue, Blue, Blue}, {"ξ≡ξ'", "η\[Rule]η'", "ζ\[Rule]ζ'"}, 
      {Green, Green, Green}, {"ξ'\[Rule]x'", "η'\[Rule]y'", "ζ'≡z'"}}]]

does not work.

I failed to find a duplicate.

Answer 1

To avoid redundant color swatches:

g1 = Graphics3D[{Opacity[0.6], Red, Sphere[]}];
g2 = Graphics3D[{Opacity[0.4], Blue, Cylinder[]}];
g3 = Graphics3D[{Green, Cone[]}];

Legended[Show[g1, g2, g3],
 SwatchLegend[
  {Red, Blue, Green},
  {Column[{"", "x\[Rule]ξ", "y\[Rule]η", 
     "z≡ζ", ""}],
   Column[{"", "ξ≡ξ'", "η\[Rule]η'", 
     "ζ\[Rule]ζ'", ""}],
   Column[{"", "ξ'\[Rule]x'", "η'\[Rule]y'", 
     "ζ'≡z'", ""}]}]]

Answer 2

Show already combines these if you include the legends in your assignments.

x1 = Legended[Show[Graphics3D[{Opacity[0.6], Red, Sphere[]}]], 
  SwatchLegend[{Red, Red, Red}, {"x\[Rule]ξ", "y\[Rule]η", "z≡ζ"}]];

x2 = Legended[Show[Graphics3D[{Opacity[0.4], Blue, Cylinder[]}]], 
  SwatchLegend[{Blue, Blue, Blue}, {"ξ≡ξ'", "η\[Rule]η'", "ζ\[Rule]ζ'"}]];

x3 = Legended[Show[Graphics3D[{Green, Cone[]}]], 
  SwatchLegend[{Green, Green, Green}, {"ξ'\[Rule]x'", "η'\[Rule]y'", "ζ'≡z'"}]];

Now simply:

Show[x1, x2, x3]

Answer 3

you can also do it like this:

Legended[Show[{g1, g2, g3}], 
 Grid[{SwatchLegend @@@ {{{Red, Red, Red}, {"x\[Rule]ξ", "y\[Rule]η", "z≡ζ"}}, 
  {{Blue, Blue, Blue}, {"ξ≡ξ'", "η\[Rule]η'", "ζ\[Rule]ζ'"}}, 
  {{Green, Green, Green}, {"ξ'\[Rule]x'", "η'\[Rule]y'", "ζ'≡z'"}}}}]]

Answer 4

Legended[Show[{g1, g2, g3}], 
 SwatchLegend[{RGBColor[1, 0, 0], RGBColor[0, 0, 1], RGBColor[0, 1, 0], RGBColor[1, 0, 0], 
   RGBColor[0, 0, 1], RGBColor[0, 1, 0], RGBColor[1, 0, 0], RGBColor[0, 0, 1], 
   RGBColor[0, 1, 0]}, 
  {"x\[Rule]ξ", "ξ≡ξ'", "ξ'\[Rule]x'", "y\[Rule]η", "η\[Rule]η'",  "η'\[Rule]y'", "z≡ζ", 
   "ζ\[Rule]ζ'", "ζ'≡z'"}, 
  LegendLayout -> {"Row", 3}]]

Or

Legended[Show[{g1, g2, g3}], 
 SwatchLegend[{RGBColor[1, 0, 0], RGBColor[0, 0, 1], RGBColor[0, 1, 0], RGBColor[1, 0, 0], 
    RGBColor[0, 0, 1], RGBColor[0, 1, 0], RGBColor[1, 0, 0], RGBColor[0, 0, 1], 
    RGBColor[0, 1, 0]}, 
   {"x\[Rule]ξ", "ξ≡ξ'", "ξ'\[Rule]x'", "y\[Rule]η", "η\[Rule]η'", "η'\[Rule]y'", 
    "z≡ζ", "ζ\[Rule]ζ'", "ζ'≡z'"}, 
  LegendLayout -> {"Column", 3}]]