Regions and their combinations

Question

I met stupid problems playing with combinations of simple 3D regions. Can anyboby explain what is wrong in my code?

I need to make a model for 3D-printing.

RegionPlot3D[{
  Cuboid[{-3, -27, 0}, {-6, -50, 25}],
  Cuboid[{-5, -41, 0}, {-20, -38, 25}],
  Prism[{{-27, -38, 0}, {-19, -38, 0}, {-19, -45, 0}, {-27, -38, 
     25}, {-19, -38, 25}, {-19, -45, 25}}],
  rg1,
  Cuboid[{-3, 27, 0}, {-6, 50, 25}],
  Cuboid[{-5, 41, 0}, {-20, 38, 25}],
  Prism[{{-27, 38, 0}, {-19, 38, 0}, {-19, 45, 0}, {-27, 38, 
     25}, {-19, 38, 25}, {-19, 45, 25}}]},
 PlotPoints -> 81,
 Axes -> True, AxesLabel -> {"X", "Y", "Z"},
 PlotRange -> {{-60, 60}, {-60, 60}, {0, 27}}]

where the rg1 is 3D-arc:

rg1=RegionIntersection[
      RegionDifference[
       Cylinder[{{0, 0, 0}, {0, 0, 25}}, 30],
       Cylinder[{{0, 0, 0}, {0, 0, 25}}, 27]],
      Cuboid[{-6, -30, 0}, {30, 30, 25}]
      ];

Generally, it looks like following:

However, I need the joint region to make the stl-file. Thus, I've tried the RegionUnion to combine the elements. However, it is not so easy:

Making the clamps, I found that simple

RegionUnion[
Cuboid[{-3, -27, 0}, {-6, -50, 25}],
Cuboid[{-5, -41, 0}, {-20, -38, 25}],
Prism[{{-27, -38, 0}, {-19, -38, 0}, {-19, -45, 
          0}, {-27, -38, 25}, {-19, -38, 25}, {-19, -45, 25}}]
]

produces an error-message:

RegionBounds::reg: Cuboid[{-3,-27,0},{-6,-50,25}] is not a correctly specified region.

Ok, I've tried following:

rc1=RegionUnion[
  Region@Cuboid[{-3, -27, 0}, {-6, -50, 25}],
  Region@Cuboid[{-5, -41, 0}, {-20, -38, 25}],
  Region@Prism[{{-27, -38, 0}, {-19, -38, 0}, {-19, -45, 
      0}, {-27, -38, 25}, {-19, -38, 25}, {-19, -45, 25}}]
  ];
rc2 = RegionUnion[
Region@Cuboid[{-3, 27, 0}, {-6, 50, 25}],
Region@Cuboid[{-5, 41, 0}, {-20, 38, 25}],
Region@
Prism[{{-27, 38, 0}, {-19, 38, 0}, {-19, 45, 0}, {-27, 38, 25}, {-19, 38, 25}, {-19, 45, 25}}]
];

It works good, making the separate clamps like following without errors:

But when I try to combine the whole model I again met the errors: RegionPlot3D[{rg1, rc1, rc2}]

RegionPlot3D::nnregion: Region[BooleanRegion[#1||#2||#3&,{Cuboid[{-3,-27,0},{-6,-50,25}],Cuboid[{-5,-41,0},{-20,-38,25}],Prism[{{-27,-38,0},{-19,-38,0},{-19,-45,0},{-27,-38,25},{-19,-38,25},{-19,-45,25}}]}]] cannot be automatically discretized.

RegionPlot3D::invplotreg: {BooleanRegion[#1&&!#2&&#3&,{Cylinder[{{0,0,0},{0,0,25}},30],Cylinder[{{0,0,0},{0,0,25}},27],Cuboid[{-6,-30,0},{30,30,25}]}],Region[BooleanRegion[#1||#2||#3&,{<<1>>}]],Region[BooleanRegion[#1||#2||#3&,{Cuboid[{-3,27,0},{-6,50,25}],Cuboid[{-5,41,0},{-20,38,25}],Prism[{{-27,38,0},{-19,38,0},{-19,45,0},{-27,38,25},{-19,38,25},{-19,45,25}}]}]]} is not a valid region to plot.

RegionPlot3D::argr: RegionPlot3D called with 1 argument; 4 arguments are expected.

When I try the RegionUnion[rc1,rg1,rc2], it gives me following:

Region[
BooleanRegion[Or[#, #2, #3, 
And[#4, 
Not[#5], #6], #7, #8, #9]& , {
Cuboid[{-3, -27, 0}, {-6, -50, 25}], 
Cuboid[{-5, -41, 0}, {-20, -38, 25}], 
Prism[{{-27, -38, 0}, {-19, -38, 0}, {-19, -45, 0}, {-27, -38, 
    25}, {-19, -38, 25}, {-19, -45, 25}}], 
Cylinder[{{0, 0, 0}, {0, 0, 25}}, 30], 
Cylinder[{{0, 0, 0}, {0, 0, 25}}, 27], 
Cuboid[{-6, -30, 0}, {30, 30, 25}], 
Cuboid[{-3, 27, 0}, {-6, 50, 25}], 
Cuboid[{-5, 41, 0}, {-20, 38, 25}], 
Prism[{{-27, 38, 0}, {-19, 38, 0}, {-19, 45, 0}, {-27, 38, 25}, {-19, 
    38, 25}, {-19, 45, 25}}]}]]

but it does not produce a normal 3D-region.

What is wrong with these 3D primitives?