Listen up people, thje problem is not with the discs. I tried 4 copies of Halo 3, all worked to some point - I got random unreadable disc errors when loading the next mission in campaign (from mission 1 to 2, 3 to 4 etc...) and almost always on multiplayer maps like Snowbound, Guardian and Epitaph (plus I had one frezze per match on Valhalla almost always).
All of those copies played with no problems on other consoles.
However I tried to remove my 20GB HDD yesterday and used a temporary profile (offline on system link with 8 players) and I could load any map with no problem. That means that the problem (and solution) lies within this storage disc. I didnt try out the cache clear method (i doubt it will work) in memory section (press y, x,x, lb,rb,x,x - perform maintenance).
I suggest that everyone experiencing same problem tries this HDD removal method and reports if that helped. However I do not intend to play without my hdd because I cant use my profile thus cant sign in XBL.
my GT: NinjaKlan