Don't have much time but:
In MY opinion:
There is stuff online about how to "pass" your three hour. UNLESS those are changes you want to make for every day of the rest of your pregnancy DON'T DO THEM. It's so important to know if you have GD or not, swing the test could be very bad for you! AND: they have changed the standards so if you fail don't get too worried, it's harder to pass now.
I bet, as so many have said, you'll sail though but in case you don't, or are worried, here's some good things to thing of!
I was the same way, thinking I had a really good diet, good foods, what could I fix? Well, it was about leveling carbs. I am very slim (well, not today! but in general!) and could not see where I could do fixes but it was all about lowering carbs and being very stead with them. I could eat one tortilla as a starch but two pieces of bread at one time was too much. I had to cut out fresh fruit except a bit with other foods. i could eat as much veg as I liked, my morning cereal, which was healthy, was way too big but fine if I had 1/2 of it and cottage cheese... You CAN have little treats, just spread them out!
So, DON'T worry, you will probably sail though, but if not I have a 100 tips I will happily share if you want them!
Most important is that they FIND OUT if you have it or not! You can save yourself and your baby a lifetime of issues by being super on top of it, really controlling your diet and doing all you can but you have to know, so better to find out!
I controlled it all with careful diet and Baby Adric has sailed though his tests and is in the clear, so VERY worth it!
Just my opinion!
hugs and good luck,