All I know about each is that they are war games. I am used to TF2 and some Battlefield Heroes (beta) so I am used to the cartoon style fps's. Tell me a bit about these three? Differences? Any good? Level of teamwork needed? Blah blah blah, let your thoughts flow my young grasshoppahz.
Teamwork, teamwork, teamwork: To succeed in this game your team needs be organized.
No more spray and pray: Need to be accurate when shooting.
Goodbye cross hairs: Must use your iron sights or scopes to be pressie.
Day of Defeat: Source
Teamwork, teamwork, teamwork: It's all about setting up, and getting people in the right spots. Choosing when to attack, and when to defend can make or break a game plan.
Spray and Pray: Most of the guns, except sniper and rifleman, are generally spray and spray guns, though still accurate.
Continuous Gameplay: This game has the same style of respawn as TF2.
Gameplay Modes: Include Control Point style maps, where you capture each flag, and also Bomb Plant maps, where you attempt to destroy objectives.
Counter-Strike: Source
Guns, Guns, Guns: Lots of weaponry to choose from, easy to find which gun your comfortable with.
Spray or precise: It's up to you how you play really, skill levels vary.
Gameplay Modes: Several different styles of gameplay including Bomb Defusal and Hostage Rescue.
Game Mods: An insane array of gameplay modification from zombie mod, to surf, to bunny hop. Anything you can think of, they've attempted it.
Everyone feel free to add onto what I've said. There's still lots to talk about.
I haven't played DoD (I'd like to), but basically Insurgency is more realistic, and CS:S is a little more "game" style, although it's still harder than a lot of games. Both are good practice for other shooting games too, as neither is very forgiving