pyqgis 3 Union multiple layers using temporary layers

Question

I would like to use the tool "Union" to union 3 different layers. This tool unfortunately can union only 2 layers so I have to run it twice.

First I run it to union the layers "Layer1" and "Layer2"

param_union = {
    'INPUT' : QgsProcessingFeatureSourceDefinition('X:/HoWGK.gpkg|layername=Layer1', selectedFeaturesOnly=False, featureLimit=-1, flags=QgsProcessingFeatureSourceDefinition.FlagOverrideDefaultGeometryCheck, geometryCheck=QgsFeatureRequest.GeometrySkipInvalid),
    'OUTPUT' : "memory:myUnionLayer1",
    'OVERLAY' : QgsProcessingFeatureSourceDefinition('X:/HoWGK.gpkg|layername=Layer2', selectedFeaturesOnly=False, featureLimit=-1, flags=QgsProcessingFeatureSourceDefinition.FlagOverrideDefaultGeometryCheck, geometryCheck=QgsFeatureRequest.GeometrySkipInvalid),
    'OVERLAY_FIELDS_PREFIX' : '' }
 
union_1 = processing.run("qgis:union", param_union)

Then, I'd like to take the output layer created as temporary/memory layer to union it with a 3rd layer "Layer3"

param_union = {
    'INPUT' : QgsProcessingFeatureSourceDefinition("memory:myUnionLayer1", selectedFeaturesOnly=False, featureLimit=-1, flags=QgsProcessingFeatureSourceDefinition.FlagOverrideDefaultGeometryCheck, geometryCheck=QgsFeatureRequest.GeometrySkipInvalid),
    'OUTPUT' : "memory:myUnionLayer2",
    'OVERLAY' : QgsProcessingFeatureSourceDefinition('X:/HoWGK.gpkg|layername=Layer3', selectedFeaturesOnly=False, featureLimit=-1, flags=QgsProcessingFeatureSourceDefinition.FlagOverrideDefaultGeometryCheck, geometryCheck=QgsFeatureRequest.GeometrySkipInvalid),
    'OVERLAY_FIELDS_PREFIX' : '' }
 
union_2 = processing.run("qgis:union", param_union)

But I got the error message: "Could not load source layer for INPUT: memory:myUnionLayer1 not found"

How can I access my temporary layer to use it in the algorithm?

Answer 1

union_1 = processing.run("qgis:union", param_union)

Should be

union_1 = processing.run("qgis:union", param_union)['OUTPUT']

Or union1 is a dictionary, not a layer