The third mission is split into two parts, each with it's own objective. The first part requires you to research and add the Power Efficiency Module to your Power Generator and give you ten minutes to do it. Don't dawdle, as waiting until the last minute will very likely result in failure. Once that's done, the mission timer will reset to thirty minutes.
Now that you've got the power flowing in, a vehicle transport aircraft will arrive at your base. The transport had ten unit slots, so load it according to your preference or with the units you have available. A safe bet is to load eight combat and two support units, providing both firepower and survivability for your initial forays into the irradiated high deserts of Arizona. Once the desired assortment of units is loaded into the transport, tell it to launch.
The transport will take the expeditionary force to a long and narrow box canyon within the area of operations. The landing zone is at the southern end of the canyon; once landed, order your units into formation and move them north through the canyon to the Scavenger warehouses where another tech cache is located. There will be Scavenger forces scattered throughout the canyon, mostly infantry and a few Chopper Trikes, and over all defense for the low ground is light.
Immediately out of the landing area will be a small corridor that leads to a plateau on the eastern edge of the canyon. This will be an ideal spot to park artillery if any were loaded with the attack force. In the center of the western face of the canyon will be a ramp that leads to another ramp at the north end that can be used as a sort of "back door" to the warehouses. This second ramp can be useful for circumventing the defensive units in the canyon.
Once you reach the northern end, two groups of Scavenger vehicles will race out of small offshoots to ambush your force. These groups consist exclusively of Technical Jeeps and Dune Buggies, which are easily destroyed but can cause significant damage if not dealt with promptly. The cache is located near the "back door" entrance and guarded by a flamethrower tower, which is easy to neutralize from beyond its engagement range.
Once the artifact has been acquired, use the back door to exit the warehouse area and race back south to the landing zone. On the western plateau will be one last group of Scavenger units that will attempt to stop you from leaving with the tech. Engage them in mobile combat and don't stop; keep moving south and take out the Scavengers as your units are able. Order all your surviving units back to the landing zone and return to base.