The corruption gets worse with time. Flushing video memory (eg. with FLUSH command) doesn't help, unlike in A2. Overheating is not likely to cause it, because the temperatures of components are not high when playing A3 (GPU less than 50°C, CPU less than 60°C, all other components less than 50°C)
Video of the corrupted graphics: http://www.youtube.com/watch?v=1gP4IU3jTjw
In Arma 2, the flashing polygons appeared often when the components were stressed more than normally. Especially driving through Elektro and getting close to the FPS killer, enterable school building located next to the Elektro-Mogilevka road junction seemed to trigger the corruption often with my old rig. Some users got it often when they flew over or went into Chernogorsk. Here's the typical flashing polygons in A2 recorded by another user: https://www.youtube.com/watch?v=1SitcAf48zA