I have 2 dataframes:

```
df1:

col1 col2 col3 

A     B    C

df2:
col1 col2 col3 col4
A     B    C    D

```

I want to update df1 with df2 such that if I match on col1,col2,col3, then replace col3 with col4

```
df_want:
col1 col2 col3
A     B    D

```

In SQL, it would be:

```
select df1.col1,df1.col2,coalesce( df2.col4,df1.col3) as col3
from df1 left join df2 on df1.col1=df2.col1 and df1.col2=df2.col2 and df1.col3=df2.col3

```