Call of Duty 3

This video shows a level where you can get the Big Air, Rifleman, and Battlefield Scavenger. I take no credit for this video, it is not my work just showing it here so others can take advantage of its awesomeness.

You can unlock the "Big Air" achievement on the third chapter "Night Drop". Simply make your way through the mission until you get the opportunity to drive "Vera". There are two stages of driving in this section and the opportunity to get the achievement appears in both. Simply keep an eye out for a wooden ramp along the way. During the first driving section you will come to a stage where a tank appears on the road and you need to drive off the road and head towards the star on your map. After driving through the gate (countryside walls and a barn), there is a ramp situated diagonal right. Drive over this and it should unlock. If it fails to unlock here then during the second driving section you will eventually head into a train styard / station. Along the tracks you will find another ramp.

It is also possible to get this achievement on the eleventh chapter "Hostage." Approach the place where you rescue the third hostage, you should notice a ramp on the left. You can use that ramp to get the Big Air achievement. You can use the ramp after you rescue the hostage.