layer1 = QgsVectorLayer('...','layer1','ogr')
layer2 = QgsVectorLayer('...','layer2','ogr')
layer3 = QgsVectorLayer('...','layer3','ogr')
layer4 = QgsVectorLayer('...','layer4','ogr')
layer5 = QgsVectorLayer('...','layer5','ogr')
layer6 = QgsVectorLayer('...','layer6','ogr')
layers_edit = QgsProject.instance().mapLayersByName('layer1' and 'layer2' and 'layer3')

sa = layers.dataProvider().fieldNameIndex('area_calc')

Traceback (most recent call last):
  File "C:PROGRA~1QGIS3~1.0appsPython36libcode.py", line 91, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
AttributeError: 'list' object has no attribute 'dataProvider'

layers.startEditing()
Traceback (most recent call last):
  File "C:PROGRA~1QGIS3~1.0appsPython36libcode.py", line 91, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
AttributeError: 'list' object has no attribute 'startEditing'

layers.dataProvider().addAttributes([QgsField ('eur1_km2',Variant.Double),QgsField('eur1_pol', QVariant.Double)])
layers.updateFields()

idx=layers.dataProvider().fieldNameIndex('eur1_km2')
idx1=layers.dataProvider().fieldNameIndex('eur1_pol')

for f in layers.getFeatures():
    f[idx] = 6
    attr_sa = f.attributes()[sa]
    attr_idx = f.attributes()[idx]
    f[idx1] = (attr_sa/1000000)*attr_idx
    layers.updateFeature( f )

layers.commitChanges()

layer_names = ['layer1',  'layer2', 'layer3']

for layers in QgsProject.instance().mapLayers().values():
    if layers.name() in layer_names:
        sa = layers.fields().lookupField('area_calc')
        # do more stuff

layer_edit = ['layer1', 'layer2','layer3']
price = 6.0
for layers in QgsProject.instance().mapLayers().values():
    if layers.name() in layer_edit:
        layers.dataProvider().addAttributes([QgsField('eur1_km2',QVariant.Double)])
        layers.dataProvider().addAttributes([QgsField('eur1_pol',QVariant.Double)])
        layers.updateFields()
        idx = layers.dataProvider().fieldNameIndex('eur1_km2')

        layers.startEditing()
        for f in layers.getFeatures():
                f[idx] = price
                layers.updateFeature(f)
                idx = layers.dataProvider().fieldNameIndex('eur1_km2')
                idx1 = layers.dataProvider().fieldNameIndex('eur1_pol')
                sa = layers.dataProvider().fieldNameIndex('area_calc')
                attr_idx = f.attributes()[idx]
                attr_sa = f.attributes()[sa]
                f[idx1]=(attr_sa/1000000*attr_idx)
                layers.updateFeature(f)

        layers.commitChanges()

    else: print('NONE')