join3.py

import pandas as pd
from io import StringIO
import io

A = """
k1,k2,a3
1,44,x
2,55,y
3,66,z"""

B = """
k1,k2,b3
1,44,ham
2,55,egg
NULL,NULL,NULL"""

C = """
c1,k2
NULL,NULL
bar,55
baz,66"""

D = """
a1	a2	a3	b3	c1
1	44	x	ham	NULL
2	55	y	egg	bar
3	66	z	NULL	baz"""

dfA = pd.read_csv(io.StringIO(A))
dfB = pd.read_csv(io.StringIO(B))
dfC = pd.read_csv(io.StringIO(C))

l_join = pd.merge(dfA, dfB, how="left", sort=False)
print(l_join)
together = pd.merge(l_join, dfC, how="left", sort=False)
print(together)