It's the middle of the night so before you say someone is lying let the staff all come back to the forum and see the note on this.  Just because I don't know what happened or can't figure out the mystery does not mean someone else can't.
The last thread was closed as a current duplicate, nothing unusual about that.  The other two should have been closed when the next picture thread was started, we often close an old one of any topic when they get large, inactive or old and a new one is begun.  
We also relatively recently got new software that merges threads, and can move posts or groups of posts between threads.  It's not that intuitive to use, and not commonly used so a mistake could have happened with it.  
Obviously the option to delete posts does exists but unless the group of posts was violating a rule it would not have been done intentionally, and as Rob said it should have resulted in a note to you.  There are reasons why that would not happen right away, Moderators have real lives too, but we do try to let folks know for just these reasons.