Been having trouble with Ike recently. My bro started playing him pretty seriously, so we just sat a played the Ike/D3 matchup for like an hour. It's a tough one, since Ike can outspace you with a lot of his attacks, and you have to respect that range. This makes it especially hard to get in and make something happen. I think Ike's biggest weaknesses are the low speed and high endlag on most of his moves. You have to use those opportunities to do some damage, but most of his attacks are pretty safe on shield. The Quickdraw attack isn't, though. So you can shieldgrab there. But if he's a good Ike, he probably won't use that move.
Quickdraw is an important part of this matchup, since it's Ike's bread and butter, so to speak. He can do a fast attack to interrupt the move, but it isn't very safe. However, don't just stand there in shield while he charges quick draw. He can also jump cancel grab, jump cancel u-smash, and jump cancel to any aerial. He will likely mix it up to keep you guessing. Usually when he charges QD, I either try to snuff it out with F-tilt, or get a Waddle on the ground so I can match his unpredictable mobility. I like to catch him with Waddle dash to grab or down/up smash, or just keep moving around with the waddle to get him to shield. Then, it's grabbing time.
If you manage to get ahold of Ike, it's best to try and get him offstage, where you have the advantage. His recovery is pretty safe, since it launches a big, spinning hitbox up before he even moves up. Get used to where its hurtbox is. Fairs are less effective here, unless you're predicting his QD recovery. It's usually best to go for bairs and try to stage spike. This works well on Battlefield and PS2 as far as I know. SHFFL bairs are the best approach option most of the time because they are faster than most of Ike's ground moves.
I'm still working on this. I may post more tomorrow. Let me know what you guys think about the D3/Ike matchup.