The name of the friend of mine is A and I can see only the mutual friends that we have, not all his friends. The name of the person that I'm not friend with on Facebook but has 3 other mutual friends is B. I can't see his friends either, except his 3 friends we have in common. How to find out if A and B are friends?
You can see the relationship between two users by using their username or Facebook id.
Use the below link if you know the username:
Where USER1 is A and USER2 is B.
If you have Facebook id instead of username, use this link:
Where ID1 is A's Facebook id and ID2 is B's Facebook id.
This is the developer way.
Use the Message function, and ask A if they are connected to B.
It will generate a search query that looks like this:
where ID1 and ID2 are the Facebook IDs of the queried users.