You can use “Dimension Jar”’s effect for zero to three cards, so even if your opponent has less than 3 monsters in his Graveyard you can still use “Dimension Jar”’s effect.
When “Kycoo the Ghost Destroyer” is on the field and “Dimension Jar” is activated, the controller of “Kycoo the Ghost Destroyer” can remove his/her opponent’s cards in the Graveyard from play.