… It’s already solved!

To the moderator/webmaster: You need to create a special category for this one: “Spoiler puzzles”! :))

Possibile situations: 1) First of all, if there was one only guy painted, he wouldn’t see any face painted, so he wouldn’t laugh in the first place. 2) If two guys were painted, they would both laugh for a moment but they would immediately realize that they were both painted (since the 3d guy is not painted) and stop laughing. 3) If they were all painted, it would take just a little longer for them to realize that (than in case 2) and stop laughing.