From what I've seen here:
Most offices/labs/doctors won't have you fast before the 1 hour, they'll just encourage you to eat a good breakfast of proteins and fewer carbs (which, if you do research, you should do anyway. Never take a test without researching it, especially while pregnant!) Some will cap the number at 140 and if your numbers are lower than that, they'll declare you without GD. However, most offices cap at 130 and will have you go on to take the 3 hour if you fail by even one point; often times if you DON'T fail by only a few points as well.
For your 3 hour, they will have you fast for 8-12 hours (I've seen some say 8 and some say 12, for me it was "Don't eat after midnight" which equaled to about 12 hours.) They'll do a blood draw before you take the test, and then every hour afterwards, so a total of 4 times. You shouldn't drink water or anything during this time as well, so bring crackers or water to drink as soon as you're done!
All that being said; normally, I'd say to ask to re-test the one hour since it was messed up. However, a HUGE portion of people I've seen on here fail the 1 hour and pass the 3 hour anyway, so I honestly would just suck it up and go on to the 3 hour. (I've also seen people fail the one hour with over 170 and pass the 3 hour no problem.)