I have tri-crossfire unlocked 6950s and you're right, it can get bit toasty on a game like Metro with all the features turned on.
That said, my 2600k system is relatively stable o/c'ed to 4.8GHz even when all 3 cards are doing their thing.
This is all done with air cooling- $30 cpu heatsync/fan, stock GPU.
wow.. tri-cf.. that must rock! and air keeps them cool? how else can you keep them cool?