Something's definitely weird with your setup....There's always some variation in temps across the cards but 30c?? Can't comment on the 770's specifically but I'm using 3 680's (which from what I gather are basically identical to the 770's) and with overvolting (1.215v) and decent overclock (1243) it's really rare to see the cards get over 50C. Granted that's on water but there's no way it explains over 50C delta.
Try getting more airflow to your cards, play with the config and let us know.
I'd say bad tim or heatsink but that should affect only 1 card...having that kind of issue with 3 new cards would be highly unlikely imho
OOPS....posted a replay before seeing all the thread...sorry.