MainChain.py

# The hexagon bin feature class and the related fields
table = 'hex_bins_MYS'
fields = ["Matsuya","Yoshinoya","Sukiya","Max_No","MainChain"]

with arcpy.da.UpdateCursor(table, fields) as cursor:
    for row in cursor:

        Mainchain = ""

        for column_no in range(3):
            if row[column_no] == row[3]:
                Mainchain += fields[column_no][0]
            else:
                Mainchain += "-"

        print(Mainchain)
        row[-1] = Mainchain
        
        cursor.updateRow(row)