Matt Barnes -- CALLS SHENANIGANS ON 3-POINT SHOOTOUT

Something is rotten in the state of Magic Mountain ... and it's the carnival game 3-point challenge ... so says Matt Barnes who says the reason he only went 2 for 5 is because it's RIGGED!!!

TMZ Sports has the video of Matt and Deandre Jordan at the Six Flags themepark in Valencia, CA a few days ago ... where the L.A. Clippers star entered the contest ... and he didn't do great.

So, what's Matt's excuse?! Well, he was laughing about it when we saw him in L.A. the other day ... before making ALLEGATIONS OF SHENANIGANS!!!

(For the record, he's probably joking ... but, still funny!)

Check out the clip.