JC Chasez -- Yes, I Went to the Wedding (Not Justin Timberlake's Wedding)

JC Chasez was one of THREE members of *NSYNC who was NOT in attendance at Justin Timberlake's wedding this weekend ... but he watched two OTHER people say "I Do" ... TMZ has learned.

Just like Joey Fatone and Lance Bass ... JC didn't make the trip to Southern Italy to see his former boy-bandmate JT tie the knot to Jessica Biel.

Instead, JC had more important plans -- his brother was getting married in Orlando, FL.

We're told JC had planned to go to his bro's wedding for over a year -- and had previously informed Justin it was a top priority he just couldn't miss.

As we previously reported, Joey spent his weekend in L.A. (it's unclear if he was even invited) ... and Lance was at an event in San Diego.