I may not understand your question correctly, but you can use `Show` to combine graphics of the same type:

    g1 = Plot3D[x^2 - y^2, {x, -3, 3}, {y, -3, 3}, 
      RegionFunction -> Function[{x, y, z}, 2 < x^2 + y^2 < 9]];
    
    g2 = SphericalPlot3D[
      1 + Sin[5 \[Theta]] Sin[5 \[Phi]]/5, {\[Theta], 0, Pi}, {\[Phi], 0, 
       2 Pi}, Mesh -> None, RegionFunction -> (#6 > 0.95 &), 
      PlotStyle -> FaceForm[Orange, Yellow]];
    
    Show[g1, g2]

![Mathematica graphics](https://i.sstatic.net/46ovm.png)


----------

Another interpretation is that you are looking for `Piecewise`:

    pw = Piecewise[{
           {2 Sqrt[x], 0 <= x <= 1},
           {4 - 2 x,   1 < x < 2.5},
           {2 x - 7, 2.5 <= x <= 4}
          }, Indeterminate]
    
    Plot[pw, {x, -1, 5}]

![Mathematica graphics](https://i.sstatic.net/KdmmS.png)