I don’t think this is entirely possible with Makie currently, but there is similar functionality for plotting isoregions that are close to what you want, see the documentation here: volume · Makie.

For your example, it would be something like

```
using GLMakie
f(r,h,x) = -x^3+r*x+h
var_range = range( -3 , 3 ; length = 100 )
F = [ f(r,h,x) for r in var_range, h in var_range, x in var_range ]
fig = Figure()
ax = Axis3( fig[1,1] , xlabel = "r" , ylabel = "h" , zlabel = "x" )
volume!( ax , var_range , var_range , var_range , F ; algorithm = :iso, isorange = 0.05, isovalue = 0.0 )
fig
```

The `isovalue`

is the RHS of your equation (here, 0), and the isorange is a range of values close to the isovalue that will be included in the volume plot.

Edit: Using some other packages, there are ways of doing this:

```
using GeometryBasics
using MarchingCubes
using GLMakie
f(r,h,x) = -x^3+r*x+h
var_range = range( -3 , 3 ; length = 100 )
s = collect( var_range )
mcd = MC( F ; x = s , y = s , z = s )
march(mcd)
msh = MarchingCubes.makemesh(GeometryBasics, mcd)
fig = Figure()
ax = Axis3( fig[1,1] , xlabel = "r" , ylabel = "h" , zlabel = "x" )
mesh!( ax , msh ; color = :orange )
fig
```