ad granth is half of granth sahib, composed by guru arjun dev ji, but was not complete, cause bani's of gur teg bahadur shaib were not yet added to it.
ad granth was taken by dhir mal and guru sahib sent baba deep singh ji with other singhs to get it back, but the dhir mal said if guru sahib is wat people say he is, then why cant he just do it from memory? and the ad granth was not given to the singhs
ad granth and sri guru granth sahib ji the same, guru gobind singh ji recited the entire granth by memory, (i think at sabo ki talwaandi) its the complete banis of all guru sahibs, along with many of other bhagats.
dasem granth are banis of guru Gobind Singh ji, that were written by guru sahib himself.
guru sahib appointed guru granth sahib as guru before he left at hazoor sahib. Thats the same reason granth sahib is in most guruduwara's over dasem granth. but there are guruduwara's that do parkash of dasem granth as well.
yes guru granth sahib is our guru only, but we need to keep respect for dasem granth too, as its filled with bani, 3 of which are composed in the daily 5 banis of nitneem. We still need to treat dasem granth with much much respect,it is written by guru sahib himself. (but was not finished by him, but thats a different topic)