Just my speculation on what this scene may be: Judging by the costume, I think that is just after he escaped the asylum with the help of Harley - he wears a suit probably because he managed to escape during a court hearing, now visibly covered in ash/dirt perhaps because of the troubles he faced while running away.   The two other 'jokers' in my opinion are imposters - rioters who try to help Arthur escape, misleading the Police cars chasing him. One of them is wearing the clown