I think the 3 best weapons in Dead Space are the:
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Dead Space 3 does have an online co-op mode for campaign. It also includes 3 co-op exclusive levels.
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Not sure about the Level 3 code, but you can enter "super" for special weapons. Or try "ship2" or "ship3" to start with better ships.
You can get the foam finger gun in Dead Space 2 by beating the game in hardcore mode. You can get it from an unmodded version of the game on any platform (PS3, xbox 360, PC) Here is some proof so people don't remove this thinking I am lying: https://www.google.com/search?q=Dead+Space+2+Foam+finger http://kotaku.com/5746431/beat-dead-space-2-and-it-will-give-you-the-finger http://deadspace.wikia.com/wiki/User_blog:CrackShot/Dead_Space_2_Foam_Finger_Weapon http://deadspace.wikia.com/wiki/File:Dead_Space_2_Foam_Finger_Gun_Easter_Egg You can now also get something similar in Dead Space 3, which is called the devil's horns.
At E3, EA stated there will be no split screen. For now, Dead Spaces 3 co-op will have to be done online. Sucks for a lot of the people who doesn't own two of the same console and games.