Avril Lavigne did something Friday night she hasn't done in more than 3 years ... take the stage.

Avril was at a Nickelback concert at the Greek Theatre in L.A. when she grabbed a mic and performed "Rockstar" with her ex-hubby and Nickelback frontman Chad Kroeger.

You may recall, they got hitched in the South of France, a month after they started dating back in 2013. It did not go well. They were single again within 2 years. 

Apparently, no hard feelings.