Monday Night Combat

Obviously, this can easily be achieved by boosting, either with a friend or a second controller. Host a Private Match, then simply allow the one getting the achievement to get three kills in a row off of you. This is easiest if the player not getting the achievement is Assassin, as she is easiest to kill. You can also get the MVP, Sacker and Grappler (if you do grapple kills). It is possible to get all 8 Crossfire achievements in one match.

This too can be done in a "Private" game with a 2nd controller. (Simply get the account you want the achievement on to go and kill the afk controller 3 times without dieing.)

See also: Solution for 3-fer In Monday Night Combat Getting three kills in a row is not hard once you can play to your classes strengths. Juice and bacon always helps.
The Assassins are very useful for hit and run techniques: get a kill (like a behind the back grapple) run away to regenerate your skills and health and come back to kill someone else.
The Sniper is your basic ... sniper (camper). His sniper rifle is powerful enough get a one-shot-headshot-kill, but only so powerful as if you hit the body it will only get the health down by a little. His powerful arsenal of the aerial flak grenade (blue skill), trap (yellow skill) and the sub-machine gun (secondary weapon) can score quick kills at close range too, once you set up for them that is. If you can see one enemy lay down a trap and a grenade quickly and hit them with the rifle or SMG, and you can easily score a kill. Camping is useful with the rifle if you can see a space where opponents will come out, even better if you can see a space where opponents will stay still (like in their base) kills can be scored easier. The sniper is at his best when playing defensively, but if your opponent is uncoordinated or overwhelmed, going on the offense can easily pick up kills.
The Tank and the Gunner both have a deploy ability giving them higher accuracy and health during that mode. While you can only turn 180 degrees, if you set yourself up like a turret (in a good position to pick at enemies but not so conspicuous as to draw all the attention) you can easily steal kills.
The Assault has a very simple automatic rifle that can kill enemies if accurate. Any experienced player of shooters can pick up kills easily with him. The jet charge and remote explosive can easily score a lot of damage in a pinch. Better yet if you can get the remote explosive on another player, and set it off near some enemies... that explains itself doesn't it?
The Support can score kills with the anti heal, the shotgun and by using turrets. Unless you have juice or teammates, he is pathetic on the offense but has an immaculate defense and can defend the base all on his own. Use the support to keep your base upgraded and score simple kills from opponents who rush in without thinking.

This is the same as most of the online achievements for this game...

All you need to easily get most of them is a second controller, if you plug it in you can guest it the same as in Halo 3 and just play against yourself in a private match to get the achievements.