Masks are scattered in various missions and episodes. Collecting them will allow you to enable extras (accessible from the start menu). If you missed a mask, you can always go back and replay the job/ episode to get them.
#1. After getting Rioichi's cane, you'll have to control him again to retrieve Bob's cane. From the starting area, turn to the right and jump across the gap to find a mask behind the large crates.
#2: Once in control of Kid, you'll have to circle around the crates by shooting the green button that closes the hatch. Climb up the pole and jump down instead of swinging to the next hanging object. You should find the mask on the platform across.
#3: After collecting all the treasures in the game, you'll have to play Maximum Chaos. This is a combination of all the previous arcade games. Once you entered the second mini-game make your way through various obstacles until you see the last portal. Before rolling to the last portal, grab the mask that's floating around nearby.
Tip: Don't enter the high score when it's game over. Instead, press start and select Restart Checkpoint, especially if you're only after the mask.