This is a great list of the known silver skulls in Halo 3 compiled at Wikia's Halo wiki.
Wikia's List of Known Silver Skulls
Blind
Level: Sierra 117
Directions: Near the first enemy encounter stay to the right. Follow the wall on around past the small building until you reach the cliff edge. The skull is on a rock right just over the edge.
Difficulty: Normal, Heroic, Legendary
Achievement Icon: No Achievement
Effect: HUD and first person arm disappear.
Grunt Birthday Party
Level: Crow's Nest
Directions: After the first objective, en route to the barracks, you will fall down into a large pipe. There will be smaller pipes coming from the sides and Drones (insect-like enemies) flying out right and left. At the end, look over the edge to see a small ledge a few feet below you. Your goal is to just drop over the edge and immediately pull back and land under the floor you were just standing on, (on the ledge). Once there, the skull will be at the end in a room.
Difficulty: Normal, Heroic, Legendary
Achievement Icon: No Achievement
Effect: When killed by headshots, multi-colored confetti flies out of the Grunt's head, and children can be heard yelling "Yay!"
Cowbell
Level: The Ark
Directions: Begin by heading all the way to the part of the level where you battle the Scarab. After defeating it, look in the middle of the desert area which you battled it in for a fairly large structure (it’s the building you see if you are at the bottom of the ramp you are supposed to go up, and turn to look in the opposite direction). Inside of this structure are several Gravity Lifts (shown below). Take one of these for later. Next, head up the huge ramp, and head inside of the building (the one with the cartographer inside). Pass through a few of the rooms in this building, and you’ll eventually come to a room with a whole lot of Explosive crates. Right after this room is a Staircase/Hallway that leads to the level below. The Skull is located at the top of the room, at the point where you turn on the staircase to head for the next part of the slope. There are four platforms above you, the Skull is on the uppermost of these platforms. Use the Gravity Lift to jump up to it, and there you have it!
Difficulty: Normal, Heroic, Legendary
Achievement Icon: No Achievement
Effect: Explosive force increased
I Would Have Been Your Daddy
Level: The Covenant
Directions: After defeating the Scarabs and going inside, get to the location with all 7 Ring Holograms. Continue until after the cutscene where the Arbiter kills Truth, then jump through the ring holograms in a specific order. That order is as follows (1 is at the entrance, 7 is closest to Truth): 1 3 4 6 7 5 4 6 5 4 5 3 4. When you jump through the final halo, the rings will start pulsating, and you will be awarded the “I Would Have Been Your Daddy” skull. It will be right next to the bridge that brings you over to the Prophet of Truth.
Difficulty: Normal, Heroic, Legendary
Achievement Icon: No Achievement
Effect: New Dialogue in Game
