I will let you know (about) what my friends say

[Margalineta]

Hi everyone!
What sentence is grammatically correct...?
I will let you know about what my friends say
I will let you know what my friends say
I will let you know about my friends answer.

To me, the first answer sounds incorrect. Can "about" and "what" be used together?

Thanks!

 

[The Newt]

"I will let you know what my friends say" is correct; we sometimes add "about" informally but it's neither necessary nor elegant to do so.

 

[gengo]

Which sentence is grammatically correct?
I will let you know about what my friends say
I will let you know what my friends say
I will let you know about my friends answer.

As The Newt says, 2 is the best, 1 is acceptable, but 3 is incorrect and should be as follows.

I will let you know about my friend's answer. (singular friend)
or
I will let you know about my friends' answer. (plural friends)

With that proper punctuation, then 3 is also correct.