From what I can tell theres no public solution to have a 3 region grid with both a locked and resizable region at the same time. The regionResize feature of the Bryntum Grid enables the resize of all regions on the grid; in our case we would like the ability of the left region to be resizable and the right region locked.
The private configurable contains a variable called resizable which is documented as:
/**
* Set to `false` to prevent this subgrid being resized with the {@link Grid.feature.RegionResize} feature
* @internal
* @config {Boolean}
* @default true
*/
resizable : null,
The functionality described seems to match what we are looking for. Would it be possible to request this functionality?