That really sucks that you're dealing with fear of flying. I've never experienced it, but I'm sure it's caused some complications for you in your life. Weighing our options, is it best to live with your phobia and not fly ever or to take a slightly larger dosage than what your physician intended? Well, that's your choice, but in my opinion, I don't think there would be any problem in taking up to 3mg Clonazepam. This is only because you've experimented with lower dosages with no effect. 3mg is enough to send many into a blackout episode.
It's good that you don't drink Alcohol. You don't want to be escorted off the plane by U.S. Marshalls and be unable to fly for a completely different reason. As, I said, 3mg Clonazepam is right on the cusp but not totally extreme. Even one Alcoholic drink would probably be enough to send you into a blackout, so absolutely, zero Alcohol. If things got really bad for you, like you feel your anxiety rising imminently as you're getting ready to board, you can take a further dose sublingually (under the tounge) which will lead to a more rapid absorption than the oral route.
Do you notice the effects of the Clonazepam as it comes on? Are you traveling with someone who can help you if need be? The last question is important. Just having someone to direct you a little bit can make all the difference. This can be figured out.