Since the purpose of your join is to transfer data from the table to the feature class, my Blog applies. If you change the lines of code below based on the comment that precedes them to match your path, table/feature class names, and field names, this code should work to transfer the data. This code is not set up for versioned SDE data or SDE data that requires an Editor session to make changes, but it could be adapted to deal with that if needed. This code assumes the concatenation of the 3 fields uniquely identifies each row in the table. If the concatenated values of the three fields in the table are not unique, only the value from the last table record holding the concatenated value from the three fields will be passed to all of the matching features in the feature class. The code could be modified to check for concatenated keys in the source table that are not unique and do statistical operations on the value being transferred, like First, Last, Min, Max, Sum, Mean, etc. (depending on the data type of the value being transferred).
from time import strftime
print( "Start script: " + strftime("%Y-%m-%d %H:%M:%S"))
import arcpy
## Change the path and source table name to match your table
sourceFC = r"C:\Path\SourceTable"
## Change the names of the three join fields and the fourth value field
sourceFieldsList = ["JoinField1", "JoinField2", "JoinField3", "ValueField"]
# Use list comprehension to build a dictionary from a da SearchCursor where the key values are based on 3 separate feilds
valueDict = {(r[0],r[1],r[2]):r[3:] for r in arcpy.da.SearchCursor(sourceFC, sourceFieldsList)}
## Change the path and update feature class name to match your feature class
updateFC = r"C:\Path\UpdateFeatureClass"
## Change the names of the three join fields and the fourth value field
updateFieldsList = ["JoinField1", "JoinField2", "JoinField3", "ValueField"]
with arcpy.da.UpdateCursor(updateFC, updateFieldsList) as updateRows:
for updateRow in updateRows:
# store the Join value by combining 3 field values of the row being updated in a keyValue variable
keyValue = (updateRow[0],updateRow[1],updateRow[2])
# verify that the keyValue is in the Dictionary
if keyValue in valueDict:
# transfer the value stored under the keyValue from the dictionary to the updated field.
updateRow[3] = valueDict[keyValue]
updateRows.updateRow(updateRow)
del valueDict
print( "Finished script: " + strftime("%Y-%m-%d %H:%M:%S"))
