3/4 mark means its 1/4 short of over heating.
We're not talking precision instrumentation here either. This same observation/question has been posted on all the Hummer related forums. Ain't no biggy. When its hot and you run the AC, its NORMAL.
There is a TSB on this, that basically says if your customer doesn't get it that when it is not in the red ther is no problem, check the guage cluster, and if it is a certain % off (showing a higher temp than actual) switch it out to a new one that reads lower. They do not bother with anything else because 3/4 is normal.