The rule

DIR does NOT mean v. to be with 3rd person singular.

The rule

3 rd person singular does not take a suffix

As in

İyiym

İyin

İyi...not iyidir ´grammatically but you can hear it used in daily speech´

´´´The -dir has more to do with generalisation, building statements based on common facts and swiching from personal voice to common voice. ´´´

Let´s read more about it

Here  from post 11, 22, 24

http://www.turkishclass.com/forumTitle_6148_-1

And more

http://www.turkishclass.com/forumTitle_3626

http://www.turkishclass.com/forumTitle_21058

http://www.turkishclass.com/forumTitle_5789

http://www.turkishclass.com/forumTitle_35722

http://www.turkishclass.com/forumTitle_8590

http://www.turkishclass.com/forumTitle_12201

http://www.turkishclass.com/forumTitle_11962

http://www.turkishclass.com/forumTitle_8590_-1

the suffixe -dir in both 3rd singular and plural person can be used in a dictionary, encyclopedia... things like that.

As we´re not one of these, we don´t give a fig about it

Toodles.