I have two files, each of them has 12 lines, fileA has 3 columns, fileB has 1 column, like the following

FileA

a 1
b 2
c 3
..blabla

FileB
A
B
C
..blabla

Now I am trying to put the content of fileB as column 3 of fileA, e.g.

a 1 A
b 2 B
c 3 C
...blahblah

I figure there should be a simple unix command can be used to do this, am I right?

quite right, paste should do it.

paste FileA FileB will give you the desired output.

paste FileB FileA will reverse it.

wow, this is great!

I am reading manpage of paste, but not quite understand what does "-" do,

why

ls| paste -d" " -

lists a dir in one column?

I responded with an answer but I misunderstood the question.
I'm taking back my answer. I did not see any option to delete my message
So edited my message